Our experience with the Co-Op in Genshin Impact - something you don't unlock until Adventure Rank 16, a kind of "meta-level" independent of characters - is that it is very, very much unlike the game proper. Not necessarily in a bad way, but because you've played so much of the game solo at that point not being able to do the things you've done for the last twenty or so hours is quite a surprise.
The game itself doesn't seem to care if you play with other people ever, as near as I can tell, which is kind of a wild ride for what seems like an MMO. That being said, even if other players can't open chests in your world lets say - no doubt a concession to its primary, almost hypertraditional single player core - you can definitely gather resources, definitely engage with the more sophisticated combat challenges in instances ("Domains"), and you can also just… explore and kick the shit out of enemies generally, which in a regular MMO might have its virtues but it's not quite the same thing here.
It's not Pokemon by any means, but its seven elements form the core of the game's interest and in multiplayer it becomes legitimately fascinating. Your party can have four people, so if you play with a single friend you each get two - which means that there's a significant opportunity to "deckbuild" a combination of elementally attuned characters. If a "wet" enemy becomes "cold," they "freeze." If a "wet" enemy is "burned," the elemental effects are consumed but the attack that combines those elements gets buffed. I'd say these are the more simple ones, and putting them together reminds me of the best parts of Crystal Chronicles. And it's not just combat, these elements are present in the world and you see their effects all the time. It's about a million times more interesting than it has any right - or even reason - to be.