What does Site Studio actually produce?

Site Studio produces .twig files and .css stylesheets. There is a specific hierarchy of template inheritance:

• The html.html.twig level template is provided by your theme as normal
• The “Master template” is a replacement for the page.twig level template that's defines at the page level layout
• A content template is the template that controls the layout of any displayable content entity (this is defined per display view on each bundle).

Within each of these templates, the site builder can bring in Drupal fields (using the fields own display template), blocks, Site Studio elements and even  component templates.

Site Studio can also produce views templates, menu templates and breadcrumb templates.