In theory a full design system has most individual pieces defined. You know what your button styles, inputs, headers, etc are and designing new layouts is often a matter of rearranging these various components — you'll occasionally need to design new ones, so that's the outlier here.
This ensures designers are using a single source of truth when it comes to existing components (which is pretty spectacular, because traditionally designers often maintain an overlapping library of static components for design purposes). It also helps hugely that I could pull in real data instead of trying to transpose some real-ish information into a static mock-up.
And of course there's still that design/developer grey area. I'm educated as a designer, but I'm extensively knowledgable about HTML/CSS/SASS — and that still only takes me so far. I know basic JS, but I'm not going to pretend that I can use React. Sometimes I still need to build static mock-ups so I can try out animations or storyboard userflows, do usertesting, etc.