There are lots and lots of youtube videos, internet articles, and such to describe what is going on in various tenet scenes, now TV tropes has some as well.
This is an analysis of the story logic of the scenes, not the physics/science. The physics is another analysis, but it is pretty clear the moviemakers focused on what looked fine backwards and forwards on camera, plus story logic, and the physics is somewhat inconsistent. It only looks at the scenes where inversion is a big part, not the entire movie.
Stable Time Loop is a common trope, but Tenet has quite a lot more of these than most time travel stories. Nolan may have just chosen to add more no matter the story, but inversion also lends itself to these. Characters can do something, an opposite time person sees it and reacts in their futre, original character past, original character react and does the original thing, and the loop forms. Inversion also has characters continuously interacting instead of jumping, giving more stuff to form loops out of. It helps that in the movie time is fixed, rather than having branching or changing timelines, so and loops or attempted changes must be stable.
Oslo FreeportOur protagonist and his sidekick search for a painting, or possibly some plutonium (hard to tell apart, those), or possibly something unknown in mysterious Sator's vault. Later, they come back to help save Kat, and are in (well, one of them is in) for a surprise...
- Earlier Protag's timeline: Plane crashes —> fire alarm goes off, halon released —> hack through locks —> find turnstile —> fight with mysterious inverted man —> run back to original location
- Earlier Neil's timeline: Same deal, but chases and catches a normal time man instead.
- Later Protag timeline: Exit container —> run to door —> blasted through door, fights himself —> turnstile —> runs from earlier Neil —> exit door, wave later Neil in —> Steal Ambulance —> pick up later Neil and Kat, drive away
- Later Neil timeline: Run to door —> Wait —> See Protag exit backwards —> enter vault, make way through doors —> evade earlier selves —> turnstile —> sneak out of vault —> ambulance
- Overall timeline: Neil turnstiles and plane crash/halon, in any order —> earlier protag/Neil pick locks while later Neil evades —> Earlier protag/Neil reach turnstile —> fight, later Neil enters around here —> Ambulance stolen —> Engine explodes —> earlier protag and Neil head back, ambulance loaded, later protag and Neil run from shipping container
- If you find yourself in an extended fight with an opposite time opponent (happens to all of us), they are guaranteed not to die or be heavily injured. Because they are opposite time to you, if you do kill or injure them, their timeline would be fight(maybe) —> get killed/injured —> continue fighting, which is impossible. Unfortunately, the same does not apply to you, you are still able to die or be heavily injured in your future. If you do kill or disable an opposite time person, it will be your first interaction with them in any fight that occurs. (See Neil getting shot for an example) Their timeline would be do stuff —>die, yours would be kill person —> do stuff.
- The fight has a whole bunch of self-generating time loops/bootstrap paradoxes, where inverted protag does something in reaction to forward protag, who did it in reaction to what inverted protag did. Examples include:
- Flipping over the back in the first corner. interted protag flipped over forward protag, pulling him suddenly and arching his back. Inverted protag was flipped by forward protag de-arching his back and pulling forward.
- Gun push down the hallway: Forward protag sees inverted protag put his foot on forward's arm, pushing his arm back and throwing the gun. Inverted protag saw the gun bounce to forward's hand, and put his (leg in first scene, arm in second scene) on forward's arm to stop him using the gun.
- Floor crawl. Forward protag pushed inverted onto the floor, stood up, and backed away from weirdly moving inverted guy. Inverted guy got shoved onto floor, and backed away from forward guy stepping towards him.
- The entire fight is one. Forward protag saw someone jump out of the turnstile, grab his neck, and assemble a gun. He than fought this mysterious person to the art room. Reverse protag was blown into a room where he was threatened with a gun and his his wound pressed. He than defended himself and fought to the turnstile. No one actually started the fight.
- Presumably, Inverted Neil got through the doors by waiting for them to open, going through, than hitting the code in reverse, or just opening them in reverse if a code isn't needed on the inside.
- How uninverted Neil and Kat get out is hard to figure out. There are some theories, but it seems he either has to time things really well, or use another exit. He than hides near the plane and leaves right after the engine explodes.
Car Chase and interrogationProtag and Neil go to steal some plutonium for Sator, while avoiding actually giving it to him. But things go...a bit wrong.
- Protag timeline: Steal algorithm (the normal time part, I'll ignore this) —> hear backwards Sator —> Sator chase and threat —> Car flip —> tosses case and algorithm —> Saves Kat —> interrogation —> Tenet arrives —-> Invert —> Goes to car, bugs case, chases Sator —> Center car in exchange —> Car flip —> exploded by Sator —> Shipping container
- Neil timeline: Same as protag until —> Save Kat —> meet Tenet —> go to freeport —> tenet arrives —> invert —> shipping container
- Sator timeline: Freeport, attacks Kat —> On radio —> pistol whip protag —> tenet arrives, invert —> interrogation —> give Kat to goons, get in car —> search for algorithm —> in car, get case —> get in Kat's car —> exchange, see algorithm tossed —> knocks over inverse protag car —> follows Protag and Neil car —> back to inverse protag car, blows it up —> freeport, pick up Kat —> further adventures
- Kat timeline: Freeport, beaten by Sator —> car with inverted Sator and inverted driver —> left in car, saved by protag —> captured by goons, brought to freeport with inverted Sator —> interrogated, shot —> tenet arrives —> inverted —> shipping container
- Overall timeline: Kat beaten —> Inverted Sator and driver arrive/Kat leaves freeport(in the same car) —> Algorithm stolen, tenet cleans up inverted protag's car (in either order, tenet cleanup may be earlier) —> Sator blows up inverted protag's car and talks backwards, in either order —> Sator's car meets Neil/protag's car —> inverted car flip —> exchange —> Saving Kat, Sator gets empty case —> goons grab Kat and protag, inverted Sator searches for algorithm —> Kat, protag, inverted Sator at freeport, inverted protag bugs case —> interrogation, Neil meets Tenet sometime in here —> Tenet arrives —> inverted tenet leave at some point —> protag gets car, Kat looked at —> protag, Tenet, Neil, Kat inverted
- Algorithm timeline: Truck —> Stolen —> case tossed to Sator, algorithm to inverted protag —> Case tossed/picked up by Sator —> Case bugged by protag —> Algorithm at car at freeport, presumably taken by Sator's goons
- More self generating time loops
- Sator deliberately runs a temporal pincer, which is one of these
- Sator's car behind Neil's car. Neil is trying to evade Sator's car that sticks on them like glue, while Sator's car just follows the movements Neil's car is making to evade them.
- Sator's finger 3, 2, 1 is why they throw the empty case when they do, Sator threatens when he does because the case was just thrown.
- In the interrogation, Protagonist answers (sort of) Sator's questions, while Sator is listening to protag's answers and asking a question that make sense in response.
- To Sator's car intimidating Neil, it would feel like Neil was chasing them in the same way Neil and protag felt chased. This is likely deliberate by Sator, who is used to operating in backwards time.
- Forward Sator must have been told by his goons that there was a case that was empty, otherwise he'd have no reason to pick it up, and would have no reason to ask protag where the algorithm "really" was. They likely did not tell him about it being in protag's inverted car, or Sator wouldn't have bothered searching at the gunfight.
- An inverted car in forward traffic (or forward car in inverted traffic) is safer than it looks. If the inverted car is still driving, it didn't hit anything to hard in the world's future. If the normal cars are all driving, the inverted car didn't hit them in the past. An inverted car coming towards you while driving normal has avoided you already in its timeline, while you driving away from it have avoided it in its timeline. Of course, if you do see an opposite time wrecked car, someone is in trouble (hopefully not you)
- Sator clearly has experience operating inverted, being very good at acting like a forward moving person. (Even though I heard the backwards lines in the interrogation, and kind of had a sense he was inverted, it still surprised me to see the scene in reverse.)
- Neil and Kat are in a shipping container while Sator is there doing his thing. Depending on the sound transmission, they may hear some of the freeport stuff. Good thing Sator doesn't check his containers.
- Sator's guys would need to get the inverted chase car from the freeport to the traffic jam (in its own time, traffic jam to freeport if forward people did it.) Ives and co would have to, in forward time, place a frozen car onto a highly and put a knocked out guy into it, which would look strange to civilians in the area.
- The inverted tenet agents have to leave (in normal time) after their original selves arrived at the freeport. If inverted tenet left at the same time normal tenet arrived, the inverted agents would have run into Sator, or cut things awfully close. They would also run a good chance of hitting their uninverted selves crossing over to get Kat, so likely left the freeport at a later time than when Kat was taken across.
Final BattleTenet and Sator's men do battle. Tenet goes all out, bringing in lots of forces, and running both forward and backward in time, to stop the algorithm from being buried and used.
- Red team timeline: Briefing —> dropped at lower level landing area —> fight through city —> Building blows up/n blows up, Ives and protag split —> fight to high ground, ives and Protag in cave —> evacuate by helicopter -> bomb explodes, Ives and protag pulled out —> Tell blue team needed info
- Blue time timeline: Briefing —> Explosion —> landing high ground —> fight through city —> blow up/unblow up building —> fight to lower landing zone —> evacuate —> tell red team needed info
- Neil timeline: Follow blue team until building blows up —> keep fighting —> see Volkov plant tripwire —> invert —> chase protag and Ives in humvee —> go to bomb site, pull Ives, protag out —> invert —> go to cave, through gate —> lock gate (unlocking in forward time) —> shot by Volkov
- Volkov timeline: arrive —> plant tripwire —> ambush protag and Ives —> shoot protag..ah crap —> fight protag, get killed
- Overall timeline: red and blue team at lower landing area —> fight in lower area and city, Neil inverts, Volkov plants tripwire a bit later —> building blows up/unblows up —> protag and Ives enter cave, tripwire goes off, Neil tries to stop them —> red, blue teams fight in higher city to/from bomb area, protag and Ives in cave —> later Neil shot —> Volkov fights and is killed, early Neil to bomb area —> red, blue teams leave/arrive at upper landing area —> explosion, Neil pulls out protag and Ives
- The two team follow the same path, one just looks like it is going backwards from an observer's view. Which one depends on your own time direction.
- If you are attacked by an inverted weapon and fire back, you do not kill the attacker if they were inverted. If you did, their timeline would be fight —> die —. fire weapon. In fact, your firing may give them the position to fire at you, a Stable Time Loop where each attacker fires at the other who revealed their position. However, a same time enemy using an opposite time weapon can be hit, as can an opposite time enemy who is in the same location as whoever fired.
- Neil's emergency inversion isn't needed, he could have just told the red team about the tripwire during the normal briefing so they could prepare. Of course, this may mean Neil doesn't help later by pulling Ives and protag out, or by opening the door and taking the bullet, so if Volkov got the jump on Ives or the door stayed locked, the plan may have failed if he did this.
- If tenet paired inverted solders or groups properly with uninverted groups, and arranged proper signals, they could avoid almost any weapon. Whichever time direction a weapon operates in, someone can see the aftermath and warn the other.
- Neil's involvement is, yes, a self generating time loop. He goes in because protag and Ives mentioned someone with them, his going in allows Ives and protag to see someone there with them and tell Neil about it.
- Protag timeline: the movie we see. Kiev —> Boat, Scientists, Priya, Michael, Kat —> Oslo —> More meetings —> Talinn —> Invert, more Talinn —> shipping container —> Oslo, revert —> invert, big tenet ship —> Battle —> Probably future, kill Priya —> further operations
- Neil timeline: Other operations —> Kiev —> Maybe other stuff —> Meet protag —> follow protag —> final battle, death
- Sator timeline: Other stuff —> Supervise Kiev —> other stuff —> meet protag —> Talinn, invert —> other stuff —> invert somewhere —> Yacht in Vietnam, dies
- Kat timeline: With Sator —> meet protag —> other stuff —> Talinn, invert —> Shipping container —> Oslo, revert —> invert, big tenet ship —> revert, Vietname yacht —> probably future, avoids Priya
- In the scientist scene, protag picks up a bullet by holding his hand the right way, the inverted bullet than flies up into his hand. From its view, he dropped it normally. This is a stable time loop, protag tried to pick up the bullet because it was laying there, it was laying there because he moved his hand and dropped it. However, another stable time loop is possible, he may hold his hand and try to pick up the bullet for awhile, but than give up and leave the bullet where it is. More important to the plot, similar loops may happen elsewhere: inverted Neil in Oslo might, for example, wait by a door for awhile assuming it will let him through, but instead the door simply sits there until he finds another way through or something else happens to him.
- Backwards time communication and a steady timeline can quickly get into "I know that you know that.." territory and lots and lots of possible time loops. Confusion over this, and the difficulty of knowing how loops will turn out, may be a part of why Tenet keeps information hidden.