While building this Drupal 9 site a few months ago we needed to handle the contact form. Normally we would use the Drupal Webform module, but that wasn’t Drupal 9 ready enough at the time. So we decided to build the form ourself, AJAX based to make it as user friendly as possible and learn a few things. We published the complete code in a working, installable Drupal module. A small explanation:

So the Drupal example module contains: