When defining a content entity type, the annotation block begins with @ContentEntityType. This declaration, and the properties in it represents the definition to initiate an instance of the
\Drupal\Core\Entity\ContentEntityType class just like all other plugin annotations.

In the class constructor, there is a merge to provide default handlers for the storage handler if it is not provided. This will always default to \Drupal\Core\Entity\Sql\SqlContentEntityStorage, as it provides methods and logic to help its parent class, ContentEntityStorageBase, interact with the SQL-based storage.

Extending SqlContentEntityStorage reuses methods required for default Drupal implementations and provides an easy method to create custom methods to interact with loading, saving, and so on.