It depends on the app you use and which relays are added to its config, I believe. So right now I’m using Dani’s which is four or five relays on the go. Which one is used for my outgoing posts and replies? Not sure. I guess the notes that I see come in from all of those five relays.

Learning on the go and interested in building a relay of my own, just to learn more about them from a developer/engineering perspective.