Pull down designs via Framelink, optionally enrich with PNG exports of nodes added as image uploads to the prompt, write out the components, test/verify via Playwright MCP.
Gemini has a 1M context size now, so this applies to large mature codebases as well as greenfield. The key thing here is the coding agent being really clever about maintaining its' context; you don't need to fit an entire codebase into a single prompt in the same way that you don't need to fit the entire codebase into your head to make a change, you just need enough context on the structure and form to maintain the correct patterns.
>The designs itself are still done by humans, I presume?
Indeed, in fact design has become the bottleneck now. Figma has really dropped the ball here WRT building out AI assisted (not driven) tooling for designers.