This thread has info: Where are the logos sourced?
TLDR: TMDB primarily but does look like sometimes it’s TVDB. Fanart.tv was mentioned but dunno if it’s being used or just had similar logos.
Also admins should have control to manage them “eventually” (it was one of the things that reps promised Admin control would be updated to manage before Experience went public but then the mobile apps were released and we’re all testing Experience clients using features without admin controls now so… 
). That topic thread has a script you can use to load your own too.
