DRUPAL API Reference: https://api.drupal.org/api/drupal/8.7.x
List of Hooks: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Entity%21entity.api.php/8.7.x
Routing structure:
https://www.drupal.org/docs/8/api/routing-system/structure-of-routes
Add an item to the menu
https://drupal.stackexchange.com/a/215404/92606
Tests
https://www.drupal.org/docs/8/testing/types-of-tests-in-drupal-8
PHPUnit
https://blt.readthedocs.io/en/latest/testing/#phpunit
Run a single test by editing local.blt.yml
Drush Commands
https://drushcommands.com/drush-9x/config/config:set/
Step by step guide to creating an admin form in Drupal 8 (befused.com)
phpunit:
- path: ${docroot}/profiles/custom/site/modules/custom/my_module/tests/Unit/TestName.php