In The Game of the Gods, Morgoth and Varda play a Cosmic Chess Game. Each game is a story. Morgoth's game piece is a Mary Sue, and Varda's goal is to remove this piece from the board. Morgoth is always losing, so we go through 30 stories, each centered on some Mary Sue.
In Kara of Rokyn, the main character is the lead actress in the "Kal & Lyla" film, whose shooting is the final story arc's central plot.
The South of Nowhere story, Romance on the Set Spencer and Ashley (and many of the other characters) star in a teen drama called North of Somewhere and meet during production.
The Skyhold Academy Yearbook series has several examples in the form of various characters writing Real-Person Fic about themselves and each other. If Varric Tethras isn't the one writing it, he's usually the one reading it out loud to other characters (and thus to the reader).
Superhero RPF is a fanfic about fanficers so it has a lot of fanfics in it. Most just referenced by their title and summaries, except For Asgardia!, by a guy going by the username Sassgardian, from which there are excerpts all around the fic. It gets him accused of Draco In Leather Pantsing for his heroic interpretation of Loki, and when his identity comes out, he is in fact Loki writing Self-Insert Fic.
Tales of Karmic Lies Aftermath features Ladybug: Miraculous Journey, an official movie based off of the titular superheroine's adventures. Marinette and the rest of the current Team Miraculous are involved with the project, and several chapters focus on the movie's development and plotline.
Notably, the project is used by several characters as a way of dealing with various issues. Nooroo, for instance, deals with the trauma of being used for Hawkmoth's evil purposes in a rather unusual fashion: he insists that his movie counterpart become The Man Behind the Man, an Evil All AlongManipulative Bastard playing Hawkmoth for his own ends. He does this specifically because he knows a Control Freak like Gabriel would take even a fictional counterpart being depicted as somebody else's pawn as a personal insult.
The movie also naturally diverges from reality in various other ways, such as how it reflects the public's belief that Adrien/Chat Noir was secretly working with his father.
The United Heroez have their own movieverse; it's mentioned in passing that not all of the entries have met with their approval, such as the disastrous Knightowl V Majestia: Dawn of the United.
"You wanna plop down and watch another episode with me, Misato? Shinji and I have been watching it lately. It's like someone heard about NERV, took a lot of drugs, and then tried to make it a comedy-drama-angst-romance with an ensemble cast of traumatized lunatics. It's a cheesy giant robot show, but a lot of fun. Everyone in it is pretty hot, too. Especially that Akram guy. Yow."
Cheating Death: Those That Lived: Panem is a fan of the Romantic Comedy sitcom, Fiona and Lawrence which is the most popular show besides the Hunger Games, and possibly one of the longest-running television shows in Panem.
Freddy Freeman: "Denny, if it's about the new comics, they don't come in today. You know that, right?" Denny Smith: "Right, Mr. Freeman. I just want to make sure you know to put the right ones back for me." Freddy Freeman: "Sure. Superman, Uncle Scrooge, Crime Does Not Pay, and Frontline Combat. They'll be here for you, Den, when they get here. If they get here."
The Ikaris: The "Super Gendo Super Show", created by the PR department to make people more aware about Commander Ikari. It is like Evangelion on crack with a healthy helping of Super Robot Genre and tokusatsu tropes. They are making lunchboxes, too.
In Junior Officers, Deborah is a huge fan of an anime called Intergalactic Imogen.
The Legend of Genji: Kalsang is a diehard Fan Boy of the sci-fi TV series Starbenders: A Journey Through Space to the point where he's seen every episode, constantly shares his fan theories with his family, and has even named his pet sky bison Aimee after one of the show's characters. Luan is also a fan of Starbenders and has personally met the show's cast.
In My Huntsman Academia, Izuku has an entire collection of comics titled, "The Adventures of the All Mighty Huntsman", which are a loose adaptation of Toshinori's various adventures. There's also a TV show based on the comics, in which Toshinori's voice is impersonated by a guy named Chris Sabat.
Oni Ga Shiku Series: There is a movie franchise called "Yakuza Sunset". Izuku and All Might are huge fans. There is a Running Gag of everyone thinking that Izuku in his hero costume (a leather semi-formal suit) looks like he walked right out of those movies (though it should be noted that the movies are not his source of inspiration).
Futari wa Hearty Pure. The Cures (and also most people at their school) are big fans of the show, and Lapinyuu becomes part of the cast, playing a Shiny Luminous parody.
The two shows that helped Chloe and Chiara's father Theodore become a trampoline fetishist, Bounce and Thud, also count.
The 3 Little Princesses: Rosalina loves the show "Comet Academy", which comes on once every 86 years. Peach and Daisy find it utterly boring.
Rosalina: 12 Lumas are sent on the Academy Comet, which is the host of the Comet Academy! During the 86 years it takes for the comet to come back, the Lumas are training by eating the most Star Bits they can, so once in competition, they can transform into the most formidable things!
In A Very Kara Christmas, Kal's Christmas gift for his cousin is a copy of "The Kryptoniad", Krypton's greatest epic poem which Kara loves reading.
In The Vinyl and Octavia Series the protagonists get a book written about their first adventure. It's implied to be badly written.
Show is a plot point:
In Amazing Fantasy, Marvel Comics, the Marvel Cinematic Universe, and the Spider-Man Trilogy all exist in Izuku's universe, cementing Spider-Man has a purely fictional character. This is why Izuku is so bewildered when he starts developing spider powers and meets Peter Parker the same day after being bitten by a genetically-altered spider.
In Equestria: A History Revealed, the Lemony Narrator writing the fic-essay decides to go out drinking and just copies a large block of test from a book on a soldier's recounting of a significant battle in the history of the Equestrian Civil War into the fic. Notable in that the copied text proves to be vastly different in tone than the rest of the tone of the fic, probably due to its In-Universe copied nature, and is even written in a more high quality fashion than the rest of the essay-fic.
Friendship is Optimal: The plot entirely revolves around the fictional videogame Equestria Online, an MMO tie in to My Little Pony: Friendship Is Magic created at the height of it's popularity. Focus is shared between the game's developers, players, sellers, and the mysterious AI contained within.
Guys Being Dudes: My Edgelord Phase Summoned The Demon King Who Made Me His Boyfriend, the movie Spark and Arlo watch at the park as part of their celebration of Spark's birthday.
In Let Us Be Your Poison, Violet and Daisy is a propaganda-based kid's Edutainment Show featuring puppets teaching kid's how to be virtuous. It runs on Atlas' state television. Each episode ends in the duo singing about respecting family. Weiss and her brother Whitley used to watch it as kids.
Mega Man X Revenge: The titular cartridge actually has a story and isn't just some cursed game like most creepypastas. The premise being a virus known as SCP-616B infected the world with Zero killing everybody at HQ and infecting X with the virus. X has to fight and defeat those infected before he succumbs to the virus himself. Of course, the game is still dangerous. After losing five lives the game warns Ivan not to get game over or he would suffer the same fate as X.
Miraculous! Rewrite: Like in the original show, the chapter "Animaestro" is centered around the main characters going to the premiere of a movie based around Ladybug and Chat Noir's adventures. However, the movie is received negatively for various reasons, which isn't helped by the fact that the director dismisses any legitimate criticism as pointless nitpicking of his "masterpiece".
A meta example with the brony fandom of My Little Pony: Friendship Is Magic where a common fandom joke is that a show exists in-universe called My Little Human which similarly has a weird cult following for ponies. Sometimes fanfics roll up with this idea, almost always for humor.
Son of the Sannin: An omake at the end of Chapter 68 reveals that there's a shonen series named Haruto which has pretty much the same premise as canon Naruto as we know it (and to the story itself). Itachi uses Tsukuyomi on an Amegakure shinobi he captured to torture him for information, by forcing him to watchthe entire filler season. Doubles also as a Take That! as Itachi describes said season as "72 hours of mediocre writing, forgettable characters, cheap animation and nonsensical yet predictable plots".
Sonic: Evil Reborn Zero: Atom the Porcupine doesn't end there. It's noted that the series was once extremely popular and rivaled another great franchise, but has gone through an Audience-Alienating Era filled with an unneeded surplus of characters and pitiful 3D games and, though getting back on its feet in recent days, is still ridiculed as a "kiddy game series."
, Starscream, Skywarp, Thundercracker, Sunstorm, Metalhawk and Pharma put on a kabuki-style play called "The Matchmaker".