> But then what if the UI changes dimensions? Then you'd need an underlying set of rules, and isn't it best if you can set those rules yourself? How would that work graphically?

Look into how various design tools handle flexible layouts. Usually a combination of flexbox/something similar to flexbox or slightly more old-school; constraints.

Here is an example with Figma: https://gdwn.medium.com/how-you-can-create-really-flexible-a...