what concept is used here ?

http://www.questforoil.com/

This site have one game in that, they using Camera view port on top of GUI how? or

they using any other techniques pls any one

what concept is used”… well, the concept is a game in which you drill for oil… (?!)

using Camera view port on top of GUI how?”… render the GUI with one camera, then render the game view with a second Camera with higher depth value. Unity - Manual: Camera component