The cinemas I worked in still used 35mm film to project the films. The projectors were huge mechanical machines that would pull the film through at a set speed and also pick up the sound from the side of the film. There is so much precision engineering involved in making and operating a cinema projector, there is the focus to think about, the racking (that’s the black lines you sometimes see at the top or bottom of the creen0, and making sure the cinema lights and curtains operate at the right times so that the audience have a good experience.
Nowadays a lot of cinemas use digital projectors which is very similar to what a lot of schools have in their classrooms to show powerpoint slides, and the film arrives on a disk. However there are still plenty that use 35mm film, and it arrives on several reels, hundreds of metres long. This is a lot more fun to operate and, I think, a lot more reliable.
(If ever you’re at a small independant cinema, ask if you can see in the projection room. Usually they will be really pleased to show you and you can see for yourself where the film is coming from)