external memory

To tack onto LImnos, yeah, most official iTunes files have digital rights management which limits what drives they can be put on and whether or not they will work.

I have a lexar micro sd card reader that can be used to transfer files and it takes some work. IT does work, per se, but the sandbox nature of iPads - no operating system and apps needing permission to communicate with each other - can make it challenging to move files around. So if you get something, practice before you leave. And practice in air plane mode because that’s often something that trips people up, just how many files need wifi to be opened and that’s something you may not have on a plane

Most flights I have been on that last 12 hours or so have a huge selection of free movies to watch.

You can see if there are third party storage solutions but I'd be surprised if you could get them to work with movies from the iTunes Store.