Comment by danabramov

A new hosting provider can preemptively request known relays to crawl it. Or relays (or apps) can lazily discover it when the user hosted there tries to log in for the first time, or their data is linked to by a known user. It’s similar to the relationships between websites and search engines.

Hosting providers don’t need to discover other hosting providers. Data only flows between hosting and apps; not between hosting and hosting or apps and apps.