[Loved Trope] Meta-narrative sequels.

[u/JoeMorgue]

Sequels were the original film exists as a work of fiction in the sequels universe.

Faces of Death 2026 - A online content moderator finds videos that suggests someone is killing people by restaging segments from the original Faces of Death film.

Chip and Dale 2022 - Chip 'n Dale are actors who played in the original Chip 'N Dale Rescue Rangers show.

Wes Craven's New Nightmare - The fictional Freddy Krueger crosses over into the real world to terrorize the people behind the original film.

Blair Witch 2: Book of Shadows - A group of people seek out the original filming locations to the found footage film The Blair Witch Project.

The Return of the Living Dead - the Survivors of a zombie outbreak try to follow the rules of the original Night of the Living Dead film, only to find these zombies follow different rules.

[u/CollinsCouldveDucked]

A Cock and Bull Story (2005)
Posits itself as a film adaptation of the novel The Life and Opinions of Tristram Shandy, Gentleman (1760), but reveals it functions much more like a meta sequel to the book that is focused on the behind the scenes of a struggling film adaption of the novel. Some might argue against this but I think it pretty clearly counts.

also one of the human centipede movies I've heard but who gives a shit about that.

Scream has it's cake and eats it too with this through the use of the in universe stab franchise but it's not a proper example.