[deltron3030]

There are others like the HIG (Human Interface Guidelines) from Apple. The ones that are in the open are more like full design systems meant for internal use and prestige. You can find a list of systems on this page: https://adele.uxpin.com

To use shadows for the z-index and layers is quite common these days though, many systems use them. Flat could be just a step after "inset" and before "raised" within a system like that, something you use for normal backgrounds and stuff that are part of the background.

The most basic element in a design system is essentially just an empty frame/box that could hold a shape and color, and optionally a shadow on a shape. This frame could become anything.