My understanding is that the curators are people with public playlists on Spotify.
We’ve probably all seen that drawing of a kid studying to chill music that everybody seems to get recommended on YouTube. They also have that playlist available on Spotify and Apple Music. My guess (with emphasis on guess) is it’s those kinds of playlists by third parties, that OP was paying a middle man to get onto.
Which is even more interesting imho. I’d get there where money exchanged in order to get on Spotify’s own heavy rotation (just like radio). Either by giving Spotify a discount per play or simply by Spotify having cost of a track as part of the algorithm that decides what to recommend.
But paying for 3rd party playlists one way or another hadn’t struck my mind at all.