I have no fucking idea what anyone in this thread is talking about. The iPhone has and has always had rounded corners. Fucking everyone phone has round corners. The only phones I can even think of that didn't have rounded corners were a Windows phone from like a decade ago and the LG V30 that nobody bought.
The device is three-dimensional. They're talking about how the sides of the device are flat planes that are exactly perpendicular to the face and rear, rather than having contour on the z axis