Brad Ellis, an Interaction Designer, writes about the corners present on the iPhone X:
Here’s where the nerd part comes in, iPhone X rounded screen corners don’t use the classic rounding method where you move in a straight line and then arc using a single quadrant of a circle. Instead, the math is a bit more complicated. Commonly called a squircle, the slope starts sooner, but is more gentle.
This is a fascinating look at the squircles seen on the iPhone and how Apple’s attention to detail comes about.
Also Read: Brad’s take on Why Reach Navigation Should Replace the Navbar in iOS Design.