One of the many changes in Drupal 8 is that all HTML output is rendered via a Twig template. This means that if you want to override the HTML for a given page, node, region or field, you can copy the Twig template that is being used to your theme and make your changes.

For any given page, node, region or field, there is normally more than one template that Drupal could use and it will choose the most specific one. So the question is, how do you know which template is being used? And if you override a template, how can you verify that your template is now used?