My first job was as a usher/ concessions worker in a large movie theater. 1234 seats. I used to help out the projectionists sometimes on my days off. Long time ago, over 50 years ago. Light was provided by carbon arc like those big search lights. Lots of other stuff goes into showing a movie. The film was 35 mm and came on reels that were loaded and threaded through sprockets and tension wheels. Just before the reel changed a bell would ring on the projector. The projectionist would fire up the light on the other projector. When the marker would show up on the screen, like a circle in the upper right corner, he would start the projector running and then a second circle would appear and he would throw a lever and a blind would close one projector window and open the window to the other projector. When done correctly the audience would never notice any difference on the screen.