Camera fps rotating question

Hi so I was wondering why, in a lot of scripts for first person camera controlled, they put Vector3.up times mouse.x. Please explain to me why it is up times x instead of right times x.

It is because the y component in euler angles corresponds to the rotation of the camera you see when the mouse moves horizontally, described as mouse.x. In other words you are rotating the camera around Vector3.up. If this is still unclear to you, try to understand how euler angles work.