The use of any secondary footage from the movie or anything else is surprisingly minimal. Once in a while there is something but the best footage comes toward the end as one person discusses the lost ending; a few set pictures are included, giving a glimpse into what had been. At the same time, we are given the story behind the ending’s change.
The documentary’s title does not refer to the mysterious staircases in the film, but rather those outside the studio. I thought the former would be discussed when I watched it. Nope.
If you’re looking for something more grounded in reality related to the film, it’s worth a watch.