is it possible to run multiple instances of an “apartment”, like in GTA5, where you can enter a friends apartment and only you and this friend are in there, while the apartment of another player in the same location, is not visible?
You would start more than one Unreal server process, and each of them would load the same level, but different players would be told to connect to different process instances.
Exactly how you manage this is up to your specific hosting infrastructure, but you’ll typically need some coordinator that allocates new process instances to not-overloaded server instances, tells them what port to listen to, and tell the clients which host:port matches up with the particular instance they’re supposed to connect to, and so on.
Also, it’s probably a good idea to have some secret password for each player for each instance they’re supposed to be in, so that players can’t join random instances by just guessing a host:port to join.
Just to add to what was saying, you can also use the Session Interface to create individual sessions with a group so that it’s easier to invite new players or join other servers together, as a group of players.