p2p cache sharing is an interseting one. it seemed to work for spotify for a long time, but they eventually found it easier to use edge caching. afaik they still use it for places with less developed infrastructure

here is a paper written by the people who designed it: https://www.csc.kth.se/~gkreitz/spotify-p2p10/

this part of the abstract gets right to the point: 8.8% of music data played comes from Spotify's servers while the median playback latency is only 265 ms (including cached tracks)