I recently got myself a new (old) iPod on eBay. It is a second-generation iPod Mini, released twenty years ago in 2005 (wow!). Naturally the next step was to load some music on there.
After some internet research and trial and error, I managed to get fusepod working, it can write files to the iPod through a simple text-based interface and read what’s already there (including playlists). It does not have an obvious way to remove files, other than just erasing the whole device and starting over. After you do this, fusepod will prompt you to create a new iTunesDB file the next time it runs. If you are curious about the iTunesDB format, see this.
It does also work fine to just install iTunes on a Windows machine (or vm) and use that to perform the transfer.
In my adventures I also tried, without success:
All of the above (and fusepod) rely on the libgpod C library, which seems unmaintained but is still widely available, including through nixpkgs and is also mirrored on GitHub as hyperair/libgpod and fadingred/libgpod. Its documentation website is down but is available through archive.org.
Finally, Rockbox replaces the built-in iPod firmware, it lets you add songs with a simple drag and drop (no software needed) but I find the UI confusing and ugly.