Directors that film on Imax generally still have a hard on for physical film.
Not that I blame them. I ran movie theaters for 20 years and while I really did appreciate how much easier my job was after we went digital, I legitimately missed working projection booth shifts when it was all film. Threading and starting two dozen projectors all day long and building prints, it was some of the most fun I ever had at a job. It was really zen, just you and the machines.
Back in the days of 35mm film, movies were delivered to theaters in individual reels. A typical movie would be five to seven reels long.
It was my Thursday job to tape all the reels together into one long piece of film for the Friday premier. I'd also have to build the trailer packs, add cues for lights down/up... just generally make sure that the movie would work as it's supposed to.
For reference, you see that platter of film in the OP picture? That platter was delivered in chunks, and building the print is putting all those chunks together to make a complete movie.
I'd absolutely do an AMA if there were a mechanism to do so.
Back in a day, movies were projected by chunks. By mean chunks, film reels, either 8mm, 16mm, or 35mm. Some reels have limited celluloid capacity to handle, so multiple reels shipped to theaters. The projectionist taking care of it, assembling the celluloid and project it using bulb.
Fun thing of this, cinemas back then "accidentally" projected those X mark from celluloid.