A while ago, I saw people suggesting Navidrome for self hosted music streaming.

I don't understand what's the relationship between Subsonic and Navidrome. Navidrome makes some use of Subsonic API. What does that mean?

Subsonic looks like a proprietary software. How does Navidrome, an open source (MIT) package, use their API? Is the API licensed for this?

Do I expect any Subsonic app to work perfectly with Navidrome? Or is it partial inter-compatibility?

Extra questions:

• Recommended android app? I would like to have good offline support to select and sync music that will then be available on my phone without network

• Podcast/audiobook support? Am currently using audiobookshelf, is this compatible at all? Not sure if any benefit even if theoretically possible.