The lead dev in my company suggested designing for mobile and desktop above 1200px. It's a new way of designing for me so I just wonder does one of you heard about this way before? He suggested that design up to 1200 px will be mobile and above 1200 desktop. When the desktop will be bigger than 1200 (ie. 15 inches screens) the margins on the size increase.
Based on the UIKit Size on Apple HIG (see https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/adaptivity-and-layout/), the largest pixel width is 1024.
So I think your lead is correct in a sense that there should be 2 designs one below 1200px for mobile and the other above 1200px for desktop. The visual estate and effects of UI elements will be significantly different for different pixel widths. This rough guide is a good enough measure to replace responsive design if there are not too many elements on your application.