Greetings,
When I first started out, articles like the one below really helped. You can make a “mobile menu” in the WordPress admin then include the code referenced in the article (replacing with your menu name) in the header.php. One thing to note, your drop-downs might not work or they will, but this is dependent on your theme. If they don’t there are a ton of articles and tutorials on creating mobile drop-downs. You can target menu items that have dropdowns by using the class WordPress gives those menu items named ‘menu-item-has-children’ and go from there. If you need more guidance on that, let me know.
https://www.wpbeginner.com/wp-themes/how-to-add-custom-navigation-menus-in-wordpress-3-0-themes/