How a Cmos not gate works
No, if the nMos is on, that means it conducts. If it conducts, it is a low resistance (ideally zero). That creates a conductive path to ground, bringing the output to that potential.

Think of the two devices as being resistors controlled by voltage. If the upper resistor is shorted, the output is 5V. If the lower one shorted, the output is at ground.