The two sites above are almost identical, but adding a sign in button greatly increases the complexity. You now need to think about user authentication and permissions. You potentially need to create an admin interface to make changes to the site. The idea behind decoupled Drupal is to use Drupal to provide the user management, database and administrative interface (among other things). You’re then free to use whatever front end technology you want to display the Drupal data.
Drupal site with a blog content type: https://dustin.su.domains/headless-drupal/
Gatsby site ingesting the Drupal JSON API: https://www.notjustin.org/import/
The Gatsby site is hosted on Netlify. Netlify monitors a GitLab Repo and automatically builds when the master branch is updated.