how to put cube above cube?

I am a beginner and I want to make ground make of cube with borders make of 4 cubes in the edges, but when I am trying to do it the cube which I want to put above for the border hide below the ground cube.
How can I put the cube above the cube of ground?
Attached image to see what I got.


Take half the cube size and add it to transform.position.y

If you post your code so far, I can add it into it if needed.