I think it looks at your contacts relays from memory.
Discussion
It’s currently hardcoded in the app but Will mentioned he’s converting it to a dynamic config pulled at runtime. Prior to December the damus.io relay was the only default.
https://github.com/damus-io/damus/blob/master/damus/ContentView.swift#L12
I believe those are the boostrap relays that are used when you create a new account.