How does FOV relate to camera lenses? For example, what would the FOV be for a 2.1mm lens versus a 2.8mm lens?

The FOV angle in Freerider is the vertical FOV.

The horizontal field of view will depend on the aspect ratio of your screen.

(So, if you´re using a full HD monitor which is almost twice as wide as it is high, the horizontal FOV will be basically twice the vertical.)

So, as the FOV depends on your screen it would be hard to give a number that would correspond directly to a 2.1mm lens for example.