It is TO.
I am indifferent towards audio quality and don’t mind missing out on a few Spotify-exclusive songs so if it is using YouTube MP3s that’s fine with me.
Is what it says in the text below my post. A lot of apps out there that download Spotify playlists pair songs with ones on Youtube to download them for simplicity. Similarly some people migrate those playlists to Deezer and download them.
From my understanding it comes down to Spotify's DRM being a little harder to bypass.