Pyramid with the top point cut off would be called what?
A frustum. Specifically, a frustum of a triangular pyramid -- as one could also have a frustum of a cone, square pyramid, etc.
What describes all cross sections of a square pyramid where the intersecting plane is perpendicular cut to base?
First, many, many people had to cut into the rock formations, and cut huge perfect stones to pile up for the pyramids. Second, each of thousands of stones had to be moved from the rock quarry all the way to the pyramid location. It took many people to build the wood structures needed to start building the halls and rooms that would be deep inside the pyramid. If getting the stones to the pyramid took…