Template suggestions

The markup used to render the header region is probably different from that used to render the sidebar region, but both are generated from the same render array.

If we inspect the source of one of the pages of the demo website, we can see that the comments for the region theme hook are slightly different from the ones for the quote theme hook:

<!-- THEME DEBUG -->
<!-- THEME HOOK: 'region' -->
<!-- FILE NAME SUGGESTIONS:
   * region--header.html.twig
   x region.html.twig
-->
<!-- BEGIN OUTPUT from 'themes/custom/alps_trips/templates/layout/region.html.twig' -->
  <div class="region region-header">

There is a new kind of comment: filename suggestions. It says that Drupal is using the region.html.twig file from the alps_trips theme (because there is a cross near the name). However, it also says that Drupal should look for another file in the templates folder...