There is, but they all come under the umbrella of polyamory. There’s lots of sub categories like “parallel” (where someone’s partners don’t have much or any contact with each other), “kitchen table” where they’re not in a relationship but do talk a lot about scheduling etc, might be friends, and then where everyone is in the same relationship or has independent relationships between everyone in a group. But lots of people use lots of different terms for those things.
Only the client. Though that’s probably enough to make sure messages leave your device suitably encrypted. Depending on the algos it could be quite vulnerable to hndl attacks, though, or (less likely) any undiscovered backdoors in the implementations. Of course, even for Signal one has to trust they’re using the public server code anyway, but at least we know they’re folding in a quantum-resistant algo.