For sure the two Scorsese reminiscences about Italian and American cinema. Oh, and his Letter to Elia [Kazan].
Not exactly documentaries, more like detailed presentations.
Obviously there are a bunch of others, as this thread demonstrates; these are just ones that immediately came to mind. So many fantastic documentaries about moviemaking…I can’t even begin to remember without getting completely lost in that rabbit hole!
Essential if you’re into Peter Greenaway. It’s not revelatory in a sensationalistic way, more a meditation on aging and creative processes.