How are multiplayer games usually set up?

I know that in multiplayer games, part of the game is clientside and part of it is the server sending the client info, although I have a few questions about how things are set up.
1.) The terrain, buildings, and other static map assets are clientside, right?
2.) Are network transforms only used on things like the player and other non static objects?
3. How easy is it to set up a test server?

In new unity HLAPI networking, setting up a server is easy task you will only send player position rotations and some other info which is important in game to server and server will forward that packets to other players.

it is not hard to setup a server i suggest you to download some example packages from asset store.