Note that you're not directly setting the size and the like. Instead, you're selecting items from a scale that should compose well together. They could advertise this better; I've read a post in the past that explained it well, but unfortunately I couldn't find it any more. https://styled-system.com might be a good place to start if you want to investigate why there might be more to it than you might assume at first.

Edit: I think this was the one: https://jxnblk.com/blog/mathematical-web-typography/