In the documentation (https://www.drupal.org/docs/theming-drupal/drupal-theme-folder-structure), they said :
Templates provide HTML markup and some presentation logic. Contrary to
Drupal 7, in Drupal 8 template files (*.html.twig files) must be
stored in the ‘templates’ subfolder.
But in my theme, I have a styleguide folder at the root & Drupal 9 find twig files in this folder.
Drupal 9 will use this input.html.twig file despite it’s not in the mytheme/templates folder.