I'd imagine that the self-published music gets discovered by a few people and put into playlists, then more people who follow those playlists discover those son

I'd imagine that the self-published music gets discovered by a few people and put into playlists, then more people who follow those playlists discover those songs, and then they percolate upwards through layers of algorithmic and editorial Spotify playlist generation including Discover Weekly. There's also the Fresh Finds playlists, which work a little differently by surfacing unknown songs that "tastemakers" have been listening to.