I lost some but not most of my movies. Same for apps. Music was the worst, but on Apple Music so not purchases (just had to re-add a lot of things to my library).

It has to do with how the individual content (film, app, song) is licensed. If the same content has a separate license in the new country, it won’t carry over.

Also note this means it will depend on the two countries you are moving between (ie. I went US > UK, and I would guess there is a lot more licensing crossover between these two countries, whereas two others may not have so much).