At this point, becoming suspicious that Apple intentionally puts things into macOS to fill up your disk over time and make you want to upgrade.

This machine lost 800mb over the last two days, even though I didn't create anything or update any software.

I went looking for stuff that might have been generated. Here's one. A directory of…copies of the desktop background images (which already ship with the OS in another directory.)

macOS creates new copies at random times. Forever. What.