Been reading the docs and it looks like you specify them in the server config.
https://docs.soapbox.pub/ditto/install#add-relays
Kudos to Alex on the documentation as well. It's very clear and easy to follow.
Ditto appears to be following a pattern in which a user's relays are simply the relays of the Ditto communities they are in. This makes sense, it's just a change in mindset because currently there are 3-4 major relays which we all connect to in hopes that they have everything (every note, like, zap, profile, etc....)
This model embraces the fact that relay consensus at a network level is not only difficult, it’s not necessary.