Playlist radio algoryithm

[Music_is_love]

Hi Folks,

Can I ask a question of you knowledgeable lot?

Context is listening to friends' playlists, or more accurately their playlist radio stations...

Exactly how does spotify populate playlist radios in respect of other people's playlists?

Does the playlist radio selection draw similar tracks based solely on musical similarity? 

Or is it more complicated than that?

Ie does it pay attention to my friends' other recent listening choices?

Or my recent listening choices?

Or some mishmash of the above? 

For a couple of friends, when I look at one of their playlist's radio feeds, the tracks listed are mostly artists I play heavily. Which leads me to conclude that the radio feature is based mostly on what I listen to, and not what they listen to.

Not interested in how useful this feature is, just in how it works. 

Thanks for any light you can shed.