Cross Platform Multiplayer?

[76561198099322879]

My partner ordered this game on their Switch
It took us a moment to realise if you want to play multiplayer you need 2 copies of the game and 2 separate consoles. I don't have a Switch myself so I bought another copy for my PC on here.
We have yet to figure out how to make it happen. We have tried setting up lobbies over LAN/Local/etc and we can never find each others lobbies. The only way I've seen to add anyone on either platform is what looks like adding Steam friends

Does anyone have any ideas at all? The only online version I've figured out is a Dukematch with bots? Feels like we're going to need to download a copy to my partner's Mac, and even then I'm sure there will be compatibility issues, right?

Would appreciate any advice or instruction.