https://www.drupal.org/docs/8/theming/twig/debugging-twig-templates

Another interesting article

https://www.betamark.com/theming-custom-block-types-in-drupal-8/

 

Prints debugging Twig

{{ dump() }}
{{ dump(variable_name) }}

https://www.drupal.org/project/twig_vardumper

https://www.drupal.org/docs/develop/development-tools/xdebug-debugger

https://www.drupal.org/project/vardumper

https://www.drupal.org/docs/8/modules/paragraphs/theming-in-paragraphs-for-drupal-8

https://www.drupal.org/project/twig_field_value