I’m surprised that no one has mentioned RKO 281, a fictional portrayal about Orson Welles’ difficulties in distributing Citizen Kane due to William Randolph Hearst’s machinations.
Baadasssss! has Mario Van Peebles recreating his father Martin Van Peebles’ making of Sweet Sweetback’s Baadasssss Song.
Adaptation is primarily about screenwiter Charlie Kaufman trying to adapt Susan Orlean’s nonfiction book The Orchid Thief into a film under contract but instead ending up writing the (mostly fictional) movie the viewer is watching; it does, however, have a brief segment with a couple of scenes set making Kaufman’s previous screenplay (Being John Malkovich) into a film.
Be Kind, Rewind is about video clerks who accidentally erase all of the VHS tapes in their store and recreate the movies by filming themselves in the role.
There are, of course, a lot of films about the making of fictional movies such as 8-1/2, Bowfinger, The Player, Tropic Thunder, State & Main, *Hail, Caeser!*et cetera. However, my favorite fictional film about the film industry is Barton Fink, which is exactly what it is like to try to write for the entertainment industry. Exactly.