group_by_parent_id example

$menu_items = wp_get_nav_menu_items( $menu->term_id, array( 'update_post_term_cache' => false ) );

        if ( empty( $menu_items ) ) {
            return '';
        }

        // Set up the $menu_item variables.         // Adds the class property classes for the current context, if applicable.         _wp_menu_item_classes_by_context( $menu_items );

        $menu_items_by_parent_id = static::group_by_parent_id( $menu_items );

        $first_menu_item = isset( $menu_items_by_parent_id[0] )
            ? $menu_items_by_parent_id[0]
            : array();

        $inner_blocks = static::to_blocks(
            $first_menu_item,
            $menu_items_by_parent_id
        );

        return serialize_blocks( $inner_blocks );
    }