Likely memberships. i was once deep in the fandom of one on her “almost” senpais, who was too short lived at Hololive unfortunately, but maybe were better outside.

Astel did it with his Mela cover too, people in the membership sent him an audio file and he picked some (not sure if all?) for the song. Also the video is all fanarts, iirc.

FYI she discusses it a little bit in 【BIRTHDAY CHAT】birthday fishe! ✨🎉🎂 #gurabirthday2024 (see timestamps in comments for the relevant section(s)!)