How to multiplayer

Core keeper was just added as a co-op game on Xbox game pass so the partner and I were like omg let's do it! We love games we can play together. We have the Xbox I play on, and he plays on the Xbox app on his computer (since I'm not seeing that this game split screens to my knowledge, lemme know if I'm wrong). How do we play together? On the Xbox, I'm not seeing any game ID if I start, or seem to be able to join a world he creates. The Xbox tells me there's no joinable worlds. If he gets it through steam, can we play that way? I don't have a computer that runs well, and significantly prefer playing on our Xbox even if my computer can run something, got that sucker precovid to do online school and if i try to run anything, it protests.