Furina and Focalor be like: "Well... Since we both gonna die why not do a classic dance for the viewers, just because... Well... We can" And then Neuvillette at the end applausing i can tell he's doing that in the background in the end of the video XD
I remember when i first watched this when i was new to genshin i actually thought it was a real cutscene 😭 i was looking forward to seeing this in the actual archon quest but its just a really good fan animation 😭😭
For anyone looking for the song, it is BGM from the game ウマ娘 (Uma Musume) in the event されば君、かなし (Sareba-kun, kanashii), and there doesn't seem to be a full version, unfortunately. Hope I helped!