Starting with iOS 7, Apple have been using nicer rounded corners in visual elements such as icons. In iOS 11, these rounded corners are abundant in system UI elements, such as Control Center and many other places.
the cornerRadius
APIs on eg CALayer
etc are more basic.
Is there some API which gives you the nice new roundedness?
see for example this question: Draw iOS 7-style squircle programmatically
also see how the answers to the question are not good; a few years onwards, is there an Apple API which gives you this type of corner curve?