I was told, the best option is to use GUI environments like GTK or Qt, but don't know how to use these programms. How do I link a calculator code with a 3D model the easiest way? In Unity?
You want a 3D calculator that works in Unity? I'm not sure exactly. I haven't used GTK or Qt, but you would basically take the code of a 2D calculator's buttons, and use it for the buttons on the 3D model.