Developers are provided with a wireframe along with contextual information for functionality. Eg: a sitemap to convey navigation. The developer codes the website to match the wireframe and tests the finished site for any bugs or performance issues.
If you’re building a custom website, the developers may build the site from scratch or use a starter template. If you’re building a WordPress website or using another CMS platform, the developer will likely customise a theme to match the mockup or wireframe.