I collected about 10 gigabytes of what I like, but I got tired of it so to speak, there were no download buttons, so I had to steal the old-fashioned way without tricks.
In fact, I’ve been trying to put together a compact portable library that I can carry around on flash drives, because I have a really bad feeling about the near future.
People are suggesting Jellyfin which is a great solution for media like music, movies, etc. If you want to have offline copies of large knowledge repositories, like Wikipedia etc, look into Kiwix.
Kiwix is a GPLv3 offline web browser. It allows offline access to Wikipedia, Wikimedia Foundation projects, Stack Overflow communities, as well as public domain texts from Project Gutenberg, and various other large, data collections. These collections are usually 100s of MB in size, downloadable with a single click, and searchable.
The Kiwix Library of available collections:
https://library.kiwix.org/
(Homepage: https://kiwix.org/)
[ Not in Fdroid yet. Can be installed via Obtainium ]
Recommended collections:
Phet Interactive Science Simulations,
BookDash children’s books,
Blender open movies.