theming – Why Drupal 9 can use html.twig files outside the templates folder?

In the documentation (, 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.
For exemple:

Drupal 9 will use this input.html.twig file despite it’s not in the mytheme/templates folder.