There are copious Nirvana documentaries, but most are made for either casual fans or sycophantic weirdos. This one from 2005, part of the Classic Albums series, trades in the typical Cobain demagoguery for an analysis of creativity, musicality, and process.
But it's long, at least by YouTube standards. Including Part 2, it comes out to 75 minutes of analyzing the making of Nevermind. I'll save you some pain and zoom in on the good parts: The first one starts at 1:25 when producer Butch Vig sits down to toggle the switches on the mixing board, revealing all the layers of "Smells Like Teen Spirit."
It's chilling to hear Cobain's naked voice minus the guitar and drum tracks, but the moments where Vig discusses creative decisions — intentionally out-of-tune guitars, whether to layer vocals, etc. — convey the extreme subtlety of creating art. The genius of the album is obvious, but the documentary illustrates the frailty of creativity: Move the knobs just a little and it's a different song.
I suggest scrubbing the timeline for Vig's face. He eventually discusses the recording and mixing of many of the album's tracks.