Servo Library

I'm watching this video: How-To Tuesday: Arduino 101 potentiometers and servos - YouTube and I noticed that he uses a range of 180 to control the position of the servo. So I am going to assume that does not directly translate to PWM.

If servos don't simply take a 0-100% duty cycle PWM signal, what exactly tells them how to position?

This signal -

Duane B
rcarduino.blogspot.com

So I am going to assume that does not directly translate to PWM.

The assumption is not a good one.
The signal is PWM (the width of the pulse defines the servo’s position), but you’re correct that it isn’t the nearly 500Hz default Arduino PWM.
The rate is 50Hz, and the duty cycle is very low, usually in the range 5 - 10%