content_entity_example
As the comment states, it looks at the defined entity (Contact) and checks the value of the annotation in Contact.php which is Drupal\content_entity_example\Entity\Controller\ContactListBuilder to see what its entity list builder class is. It knows this because content_entity_example_contact resolves to the class, because it has an id annotation defining it as such.

See: https://cgit.drupalcode.org/examples/tree/content_entity_example/src/Entity/Contact.php