Do you want to export a menu from one Magento 2 site and import it into another or simply in the same one? In this tutorial, let's walk through detailed steps on how to do this with Ninja Menus, a drag & drop Magento 2 mega menu extension.
https://www.magezon.com/magento-2-mega-menu.html

1. Open the edit page of the menu that you've already created using Ninja Menus, then open Menu Builder section.
2. Click the shortcode icon in the top bar. Then copy the shortcode in the popup appearing.
3. Create a new menu, then similarly open the Menu Builder and shortcode section. After that, paste the shortcode you've just copied in the shortcode section of the new menu.
Click Save button and the menu will be imported into the builder section.