It's easy to understand how this drone can move forward, backward, and sideways by changing the speed of the motors.  But, can anyone tell me how it is able to rotate on a vertical axis?  I don't see how simply changing the speed of one or more of the 4 motors can make it do this.

Here's an article on the basics of how drones fly...

https://www.wired.com/2017/05/the-physics-of-drones/