To simplify its a tar with jpeg2000s as an image stream, plus audio and some metadata.

Its then (usually, but it is optional) encrypted to make sure that its hard to copy. The keys are sent directly to each projector to allow time limited, and or other limited use.

The reason why its used is because it optimized for quality, rather than size. It also allows custom colour spaces and other tweaks to maintain/enforce colour correctness/image quality.

Cinemas don't use consumer grave video compression like h265, instead each frame is a jpeg 2000 image. It's heavy.

The data format used to send movies to cinemas.