Mushroom ID

I was confused as these two person are on different sub-domains of bsky.social (url after the @ symbol). Does this mean they are on different instance? AFAIK most mastodon server I see have different domain (specifically, different combination of top level domain and second level domian).

EDIT: I see, theur user name is the subdomain, and things before @ is their display name. Not the most conventional system, but it makes sense.