They're not on demand movies. I think it varies slightly between the two, but you'll find theres a Pixar channel, Marvel, Disney classics, Live action, Disney Junior, Disney Channel... etc...
They just rotate through, so unless you're onboard and watching long enough to work it out, you won't know whats on or whats coming next until you're watching (but things like Toy Story 1,2,3 seem to follow each other). While you cannot choose exactly what you want, there are quite a few channels to flick through to find something you like or something that is just starting.
There should be a list of the different channels in the cabin, or just call guest services and they can get one for you.