What happens when only high resolution layouts are defined?

I have been handed a project that is oddly constructed. Almost all the layouts are in the res/layout-xhdpi/ and res/layout-xxhdpi/ directories. The normal res/layout/ directory is practically empty.

So I'm curious as to what precisely happens when low-res devices run this app. Are the layouts simply scaled down, or is something more complicated going on?

The answer may mean a lot to how much work I need todo while organizing this project.

NOTE: At the moment, testing on low-res devices is not feasible: the project only runs on a limited number of devices. I'm just planning for the future (where we may support low-res devices).