Page template for specific path: page--about.html.twig
You don't say if /about is a different content type or just a node.

If it's a node, you need to use the format:

page--node--1.html.twig

Replace 1 with the about page node id.  See the d.o. page for reference info.

If you are trying to have a template for an about content type.  You need to add code to your theme like this:

function THEME_NAME_theme_suggestions_page_alter(array &amp;$suggestions, array $variables) {
  // Add content type suggestions.
  if ($node = \Drupal::request()-&gt;attributes-&gt;get('node')) {
    array_splice($suggestions, 1, 0, 'page__node__' . $node-&gt;getType());
  }
}


See this d.o page for more info.