That is what designers do, yes. They typically do not (and are not expected to) create the actual HTML and CSS, that's the job of the front-end engineer.

Like architects designing absurd but original buildings.

Yep, then it's on the structural and civil engineers to pare it down until it gets to an actually physically possible state.

Figma mocks give most of the info you need to map to HTML/CSS pretty easily, much more so than image mocks

It tells you the colors and will show you spacing etc