They don't see it. The relays don't talk to each other, so you have to subscribe to common relays for other people to see your posts (or notes as they call it here)
Discussion
hm, that's quite a drawback in the case of relay.mostr.pub, which is a bridge to Mastodon. it's far less effective as a bridge if nothing from there is even possible to share to people who aren't using it.
but as i understand it, i can rebroadcast anyone's message that is visible to me to all the relays i'm connected to. would i have to do that prior to sharing so that people will see it?