The movies are outdoors close to the pool area. They set up lounge chairs so people can watch the movies under the stars. Free popcorn too.
You won't see many (if any) ships with indoor movie theatres. The space can be better utilized for venues that more people will attend (and that will generate on-board revenue)
When we were on Carnival Miracle they set up a movie screen in the Mad Hatter's Ball Lounge to play "Animal House" on Toga Party Night. On Disney, they show some Diney movies in the Walt Disney Theatre.