Likely the same as PC does when a player tries to join a server with mismatched mods. If the mismatched mods are client side or lightweight, it'll probably work just fine. If the mismatched mods aren't client side and contain larger standalone content or changes, it's likely joining outright won't be possible. Modded lobbies might not even appear to crossplay players.

Console players will likely be unable to use mods. Outside of some edge cases with things like Bethesda's games, consoles don't have access to mods and the companies behind consoles don't particularly appreciate modding seeing how it's essentially file tampering, which last I checked is typically considered a breach of user agreement on most consoles.

As for automatically downloading mods? Fat chance, that's a whole can of worms with a ton of problems and security concerns.