How do PlayerState, GameState, and GameMode work?

I am very confused as to how these classes work.

I am trying to learn how to make RTS games, and I fail to see the point of replication. If my goal is to have two players against each other, like chess, would I even need to replicate the board? Couldn’t I just have two players with separate cameras playing against each other?

And if I did need to replicate them, how would I replicate just my main class so that there are two players?

My main class is the camera and controls all of the units. I want to make sure it can only control its own units. Couldn’t I just give each unit a teamid and not replicate anything?

PS I know nothing about networking or multiplayer so if I sound like an idiot feel free to correct me. Thanks!