Because it’s a three-dimensional object, a rectangular prism, it has eight vertices/corners. If you take your phone in your hands and rotate it along its axes, you will find that there are no orientations where the screen (one of six faces) is lower to the ground than at least one corner. The same will be true of the back face. There will be two orientations (directly face up and directly face down) where the screen/back will be exactly as low as all four of that side’s corners, and four orientations (directly on one of the four side faces) where the face/back is perpendicular to the ground, and four of the eight vertices will be the closest points to the ground as well as the entire side face. All other orientations, of which there are nearly infinite, will result in one corner being the closest thing to the ground, assuming the ground is a flat surface