Accessing Custom REST endpoint with rest_do_request()

I done a bit of experimenting and in my custom endpoint i replaced :

wp_send_json($items);

with :

return new \WP_REST_Response ($items);

However I do not see how to extract my data from the WP_REST_Request object. I can see it is there with var_dump. But I can find no documentation as to how to access the data. This started because of a Question in the WordPress REST handbook suggests you can access REST endpoints from PHP within a plugin. I though it would be better than directly accessing the DB. But I’ve wasted enough time.