When working with content entities and the POST method, the endpoint is different to the one used for GET requests. The \Drupal\rest\Plugin\rest\resource\EntityResource class extends the \Drupal\rest\Plugin\ResourceBase base class, which provides a route method. If a resource plugin provides an https://www.drupal.org/link-relations/create link template, then that path will be used for the POST path.

The EntityResource class defines /entity/{entity_type} as the create link template. It then overrides the getBaseRoute method to ensure that the entity_type parameter is properly populated from the definition.

The EntityResource class will run a set of conditions for the request. First, it will validate the POST request by checking whether the entity is null. Then, the current user is authorized to create the entity type if the current user also has access to edit all fields provided, and finally, it checks whether an identifier was passed or not. The last condition is important, as updates are only to be made through a PATCH request.

If the entity is validated, it will be saved. On a successful save, an empty HTTP 201 response will be returned.