Wordpress get all nav items and sub nav item by registerd slug in the code uing register_nav_menu

get menu items by slug

//get menu items by code registerd slug
function get_menu_items_by_registered_slug($menu_slug) {
    $menu_items = array();
    if ( ( $locations = get_nav_menu_locations() ) && isset( $locations[ $menu_slug ] ) ) {
        $menu = get_term( $locations[ $menu_slug ] );
        $menu_items = wp_get_nav_menu_items($menu->term_id);
    }
    return $menu_items;
}