Debugging can be enabled to inspect the various template files that make up a page and their theme hook suggestions, and check which are active. This can be accomplished by editing the sites/default/services.yml file. If a services.yml file does not exist, copy the default.services.yml to create one.

You need to change debug: false to debug: true under the twig.config section of the file. This will cause the Drupal theming layer to print out the source code comments containing the template information. When debug is on, Drupal will not cache the compiled versions of Twig templates and render them on the fly.

There is another setting that prevents you from having to rebuild Drupal's cache on each template file change, but do not leave debug enabled. The twig.config.auto_reload boolean can be set to true. If this is set to true, the Twig templates will be recompiled if the source code changes.