Few answers for you:-
Regardless if you do a seamless world or zones with a loading screen, you will need multiple servers. Each server will work for an area or zone, to coordinate movement (replication in UE speak). Seamless will be a better experience but harder to implement. On top of that other things will need to be separated into different services. Things like ingame Mail, chat systems don’t need to be implemented in the main server code.
You will need to add C++ code to unreal there isn’t a blueprint or addon way to do what you are asking.
Check out Photon Server SDK as well as the UE4 example to see how this sort of thing is done in UE4.
If you haven’t read this article check it out, it will give you some idea of what is involved.