I'm sorry I am not a native speaker, but could you allow me to put another posting?
I don’t know where they sell tickets in a cinema theater in your country. I don't watch movies in cinema theaters lately. When I was a kid, in most of the cinema theatres in Japan, the ticket counter was located at the door. But, of course, the location will vary with place and
The standard expression is 'He bought his ticket at the door'. I guess the thought is that there is a door somewhere in the vicinity, otherwise how will you get into the theatre? But questions about the door are irrelevant. Don't try to interpret the phrase too literally.
You wrote, "I guess the thought is that there is a door somewhere in the vicinity, otherwise how will you get into the theatre?"
The show place now is called a Pavilion. It used to be called an arena, is an indoor sports stadium. Its entrance has a slight walking distance from the "purchasing windows."