There are two techniques of collision detection: intersection and overlap testing. I would like to know what is the one implemented by Unit3d.
Unity uses PhysX by nVidia as its physics engine. I haven't heard of the two distinct categories you refer to, but your search for an answer could start with trying to find out more about PhysX.
If you want people here to have a better chance answering the question, you should provide references for where you found the definitions of "intersection testing" and "overlap testing" as it may not be a universally recognized categorization.
I found these definition in the book: "Introduction of game development", Steve Rabin, 2nd edition