How do I manipulate ALL players cameras?

I know how to manipulate my OWN camera on test mode. But I don't know how to do ALL's (I know you have you use Local Script).

Put a local script in StarterGui. This will make it so that the local script is copied for each player, thus letting you affect each player. As for getting the camera, you just use game.Workspace.CurrentCamera.

Since you already know some camera manipulation, I do not believe I need to explain how to actually manipulate the camera.

Hope I helped!