This crossover retroactively explains Lucifer's lack of concern for the consequences of God and Goddess potentially getting into conflict during Season 2 of his own show: he has already been through the successful destruction (and return) of the entire multiverse, so their fighting could hardly be any worse than that.
Lex dismissing the idea of the Clark Kent being Superman in his own universe, it perfectly encapsulates Lex's arrogance that he can do no wrong and is always right. Because obviously the smartest being in the universe would have been able to see through such a simple disguise easily, thus Clark Kent can't be Superman.
Earth-96 Superman is the paragon of truth and the greatest Superman makes perfect sense because he's supposed to be the Superman from the Donner Superman films and the Superman that made us all believe a man can fly. This Superman is heavily idealistic and the character at his most hopeful.
Ryan Choi at first doesn't want to be a hero, instead wanting to rush home to his wife and newborn daughter. Of course he does—he's the Paragon of Humanity. What could possibly show his "humanity" more than the urge to be with his family as the world ends, but to ultimately step up to save the universe?
Mar Novu mentioning that he used to have a family explains why he was so patient with baby Jonathan crying.
Lex's brief exposure to acting as a hero and the resultant Good Feels Good helps explain why in the rebooted universe he's made himself a noted philanthropist and highly regarded individual whereas on Earth-38 he was an unrepentant supervillain; he's put himself in a situation where he can honestly do good if he feels like it.
The new multiverse was created thanks to Oliver Queen's sacrifice. Thus, it can truly be called the GreenArrowverse.
In the ending, Earth-96 Superman is seen orbiting around his earth with his crest reverted back to yellow, and it is later confirmed by Word of God that this symbolizes that all of his deceased friends and family have been resurrected Post-Crisis. Who would do that? Kara. The Paragon of Hope and his cousin from another universe.
Why were Smallville Clois so blase over the end of everything? Because in the Season 11 comic (which was confirmed to be in canon), they already dealt with an infinite multiverse Crisis similar to this one.
Why is the Anti-Monitor's army all One Hit Point Wonders? Because they're made of antimatter, so any contact with a being made of matter would annihilate them! (Of course, it would also annihilate the person who struck them in a gargantuan explosion, but hey.)
The tie-in comic revealed that the Lynda Carter Wonder Woman tv show took place on Earth-76, which is odd considering how the Wonder Woman '77 comics crossed over with the Batman '66 comics, which are canon the Adam West show set on Earth-66. However it is likely that these two universes were merged post-Crisis like with Earth-Prime, and it would also explain away the different versions of Two-Face from the comics and the animated film. The animated films were canon on the pre-Crisis Earth-66, but post-Crisis the comics were made canon.
The inclusion of certain characters in the Justice League: While many other superheroes could potentially be members, those who start out there are either solo heroes (Clark, Jefferson) or leaders of teams/groups (Sara with the Legends, Barry with Team Flash, Kate with the Bat-family, J'onn with the DEO, Kara with the "Superfriends.") If any other big threats come up, the solo heroes will benefit from having other heroes to team up with, while the leaders can bring their teams' resources to bear on large problems, as well as calling in their allies in turn.
As an in-joke/fourth wall lean, several characters keep referring to the events in this adventure (and prior ones) as a "Crossover." It appears that this term has mutated in the multiverse to mean "hero(ic) teamup."
Fridge Logic
Fridge Horror
While it's nice (for the audience) to see the classic, and newer, actors back in roles that made them famous, one tends to forget that on each of the Earths that doesn't know about the multi-verse, or have a possibility of escaping (a la Earth-38), it is literally the end of the world as they know it. Dick Grayson on Earth-66's "Holy Crimson Skies of Death!" takes on new meaning after this.
Earth-75 even lives through the worst part of the Superman: Doomsday storyline, as evidenced by the TV picture that Iris, Clark-38 and Lois-38 see, and then it's wiped from existence.
Fortunately, they're all returned to existence with nobody remembering these events except for the seven Paragons, Lex Luthor, the Monitor, the Anti-Monitor, and the Spectre. Just a handful of beings in the entire multiverse.
Lex is now seen as a hero, so now that he is potentially a future Villain with Good Publicity, and lots of influence, think about how much damage he could do if he ever turns bad.
Lex might be too blind to realize Clark is Superman but not only does he know Kara, but thanks to being with them for Crisis he also knows the identities of most of the other main heroes. Sara mostly lives outside of time, Choi is just a random guy and Ray was already public so that's less of a problem for them but Kate and Barry do keep secret identities and Lex now has that knowledge.
For all we know, the anti-matter wave didn't just hit what was depicted on-screen. It could've annihilated tons of universes, and not just DC-based ones either. All the other CW shows, other WarnerMedia properties (shows from HBO, Cartoon Network, Hanna-Barbera, Rooster Teeth, NetherRealm Studios, etc.), and Monitor-knows what else got hit by the wave. Imagine all your favorite characters from basically any piece of media, going about their lives...then the skies turn red. They watch their worlds end...and they may not even be restored.
Early on Flash season 1, the Reverse Flash says that for him originally (as much as that term can apply to a universe with near constant time travel alterations going on) Oliver Queen lived to be 86 years old. This means that the Timey-Wimey Ball over at the Flash and Legends series, including several caused by Barry, directly resulted in Crisis coming to pass. Barry himself was at least partially responsible for Oliver's death!
