Well, either way can be done, it's pretty much up to the client AND your relay setup. Example, my client RelayTools-android, it connects to just one relay at a time. The topics feed and all feeds, are only being pulled from the selected relay. This is an efficient way to use nostr. In addition, I have a personal relay that streams in from 60+ relays, my lists and any other rules I want, assembling my view. So if I want it to pull certain topics only from certain relays, I can do that. If I want to pull them from my follows I can do that too. At all levels you get to pick, is my point.. A customizable and portable topic has no direct 'tie' to a specific relay. It is just kind11 events.