Muzan is observing the Hashiras doing their normal shenanigans
Michikatsu: He's trying to train Kaigaku but slowly failing, he's not that great of a mentor
Kaigaku: He's trying to understand what Kokushibo is saying, but doesn't understand a single word, so sometimes he's randomly swinging his nichirin blade, but most of the time doesn't do anything
Douma: He's currently annoying Hakuji
Hakuji: He's getting angrier by the second from Douma
Hantengu, Sekido, Aizetsu, Karaku, and Urogi: They're just chatting about things; Opinions on the other Hashiras, missions, etc...
Nakime: She's playing her biwa as usual
Gyokko: He's randomly drawing on scrap paper he found in his room, anyone knows where he got them? Not really
Gyutaro and Ume: Doing normal sibling activities
Enmu: Doing anything he can do, primary things with trains
Rui: Fantasticizing about a family