[ziddoap]

The title card of the video is "Please don't sue me", so I assume OP is at least somewhat familiar with the risks.

[amiga386]

and then goes on to say he lets people put in Spotify links to add songs. Spotify won't let you download songs, but he uses their API to get the band and title... then searches for it on Youtube and downloads the song from there instead

PFFFT that's the sound of Youtube's lawyers spitting out their coffee and sprinting back to their desks