I wanted to know if there was a way to mirror models/prefabs in Radiant or just in general. There seems to be a lot of models that appear to be able to be mirrored but I have no idea how to do so. Thanks for any and all help!

Do you mean like the double door models where there is a left side and right side?

Anyways, if you want to easily mirror something pretty sure there is a button near the top bar of radiant that looks like this:
X|X or Y|Y or Z|Z (Except it is a icon, not text)

Those should mirror whatever you want on the desired axis