In The Office (US version) it is revealed that the mockumentary does exist inside the universe. Dunder-Mifflin is even interviewed as stars of the show and Pam has a thing for the boom mic operator.

Netflix’ Tick tick boom is based on an eponymous theatre play, which told the background story of the shows creative process. It’s mein character is a fictions version of the creator.

Season 4 of the Arrested Development revolves around Michael getting release rights from the family members to create a show based on the story. He even hires a fictitious version of Ron Howard who is the real show’s director.

Marvel’s Shehulk ends with the main character discussing the production with a fictionalised version of Kevin Feige.

These are for examples that I could come up with from the top of my head. Can you think of any other?