Templates are the default option for hook_theme: https://www.drupal.org/node/2231673
Advanced Theme Settings Change Record: https://www.drupal.org/node/2382645
Drupal 8 theming guide: https://www.drupal.org/theme-guide/8
Templates are the default option for hook_theme: https://www.drupal.org/node/2231673
Advanced Theme Settings Change Record: https://www.drupal.org/node/2382645
Drupal 8 theming guide: https://www.drupal.org/theme-guide/8