Source

File: wp-includes/rest-api/class-wp-rest-server.php

public function envelope_response( $response, $embed ) {
		$envelope = array(
			'body'    => $this->response_to_data( $response, $embed ),
			'status'  => $response->get_status(),
			'headers' => $response->get_headers(),
		);

		/**
		 * Filters the enveloped form of a response.
		 *
		 * @since 4.4.0
		 *
		 * @param array            $envelope Envelope data.
		 * @param WP_REST_Response $response Original response data.
		 */
		$envelope = apply_filters( 'rest_envelope_response', $envelope, $response );

		// Ensure it's still a response and return.
		return rest_ensure_response( $envelope );
	}