Source

File: wp-includes/rest-api/endpoints/class-wp-rest-controller.php

protected function add_additional_fields_schema( $schema ) {
		if ( empty( $schema['title'] ) ) {
			return $schema;
		}

		// Can't use $this->get_object_type otherwise we cause an inf loop.
		$object_type = $schema['title'];

		$additional_fields = $this->get_additional_fields( $object_type );

		foreach ( $additional_fields as $field_name => $field_options ) {
			if ( ! $field_options['schema'] ) {
				continue;
			}

			$schema['properties'][ $field_name ] = $field_options['schema'];
		}

		return $schema;
	}