ATproto federates in a very different way than Mastodon. There is no concept of "instances" on ATproto.

Your account is hosted on a PDS and you sign into the app with your PDS sign-in and records go to your PDS, but everything on the app is from what's called an "AppView" which provides a centralized view of all data in all PDSes so it feels just like you're using a regular centralized app. But there can be multiple AppViews and AppViews can be self-hosted.

So unlike with Mastodon, it doesn't matter what PDS "instance" you're on because the app layer is completely separate from it.