Using the Subsonic API with great imagination :D
For the Instant Mix and Radio Stations section I use the getSimilarSongs endpoint to generate a mix starting from an artist, album or song. For the Best Of section I use the endpoint getTopSongs, while for the new releases I take all the albums released during the current year, sorted by date added.
If I had to write APIs from scratch, I wouldn't have done all this, but having to take advantage of the fame of Subsonic's APIs, I couldn't do otherwise.
Hi, I'm working on it. As for F-Droid, I think I can't upload the current version because at this moment I include play-services-cast-framework for the cast functionality and in the future Android Auto. I'm studying it, I don't know what to do yet: I could eventually publish the full version on the Play Store and the version without non-free stuff.