Thinking about both processing time and the difficulty of sustaining 120M downloads' worth of programmatic access, I wouldn't be surprised if this is actually trained on the track previews.

I’m almost positive it is. If you put in a song with a bunch of different styles, sections (e.g. bohemian rhapsody) the suggestions match the preview

> so it has to be some collab with iTunes.

There’s no way today’s Apple would allow such a collaboration. They’d just keep the feature and market it as part of Apple Music.