Hi Ismail @shola_expert ,
If I understand it correctly, you are wondering how Paul was able to place a Sphere exactly on top of a Cube.
Well, he knows how big a (default) Cube is (2m) and how big a (default) UV Sphere (2m). Adding a Sphere in the same location as the Cube and then moving it straight up 2m (½*2m + ½*2m) will place the Sphere exactly on top of that Cube.
But maybe that's not what you are asking. Please add a timestamp, when asking a question about a Lesson and if possible (a) screenshot(s).
also... you can often place it using snap to surface, edge, or vertex, which also wouldn't require measuring.