The AppView doesn't do that only for Bluesky data. It does it for any Personal Data Stores (user accounts with all their user data) that it knows about.
When you "interact" with users elsewhere, all you do is generate new records on your own PDS. You generate a "like" entry, or a reply, on your own PDS. It's your pds, all your stuff goes there. The AppView sees that and indexes it, attaches that like or that reply in the AppView to the post you're reacting to.
The AppView doesn't do that only for Bluesky data. It does it for any Personal Data Stores (user accounts with all their user data) that it knows about.
When you "interact" with users elsewhere, all you do is generate new records on your own PDS. You generate a "like" entry, or a reply, on your own PDS. It's your pds, all your stuff goes there. The AppView sees that and indexes it, attaches that like or that reply in the AppView to the post you're reacting to.