Wanted to clarify that if you hover over Euler in Quarternion.Euler it has a syntax of float x, float y, float z.
We put mouse x input in the Y parameter and the mouse y in the X parameter. This confused me for a bit until I read the help text more carefully. It is the rotation around said axis.
So the mouse input on the x axis (left and right) will return values as to how much to rotate around the y axis. Since the Y axis is vertical, rotating around a vertical axis would produce the effect of turning left and right. Same with the mouse Y.