Where I live now it's illegal for cinemas to stop you from bringing food in, so I occasionally might bring my own snack, but that might be like one out ten times. Mostly just watch the movie and eat before or after. (Spain if you want to know)
You could maybe get some sweets or chips or peanuts or similar (small snacks) but not the giant popcorn or sandwiches/hotdogs/etc., but the general idea was to go out and have (say) a pizza and a beer and later go to the cinema or - more seldom - go to the cinema and later go to the pizzeria or restaurant.
I always thought that the eating and drinking in the cinema was a US only thing, France and Germany (AFAICR) had also none of that years ago, but that maybe has changed.
Curious if in other EU countries it has ever been a thing.
In Spain you usually just buy the stuff, then go to the theater though, after some legal kerfuffles the current state of things is that an establishment can't sell you chips AND prevent you take your own chips, so we are free from absurd prices. I'm not sure that's the norm elsewhere.