Why do we mostly use triangles to represent 3D

speed, simplicity, convention
We use triangles because they provide a fast way for a computer to represent surfaces, they’re pretty simple structures, and we’ve been using them for quite a while, so computer hardware is optimized for them.

make a cube
go to gameobject and choose cube.
select move tool to change dimension

now go to GameObject and select sphere
now how about cylinder

Game Object -> 3D Object -> Cube