1

In games like Halo or CoD you can recognise, that when a weapon is laying on the ground it is just a low-poly model and when you take it, it changes into a detailed version. But the other players can only see that the other characters are holding a undetailed version of the weapon. But how can I make it, that the other players see other things than I?

2

Just one possibility: You give each player a variable that holds the players current weapon. Then you can access it from the player himself (the one with the FPS view) and tell unity which weapon to draw, as well as the other players can access it and tell unity which weapon to draw in your player models hands.