the .bsky.social is the main instance. The bridge acts as another instance.
For example one of my friends has a ATProtocol account hosted on the fellas.social instance, so his username is @johndoe.fellas.social
it basically works like lemmy and mastodon in that regard
The main difference with lemmy/fediverse is that instances don’t actually host the software, they just host the database. So it’s plug and play into any open source ATProtocol software. This dramatically decreases server loads and makes hosting an instance or your data cheaper but also means that software is more intensive to host.
This wikipedia page is true gold lol: