Adding a menu to template using shortcode

What do you mean by “into a template”? Are you asking how to add a menu item programmatically? If so, I’ve done it by building the menu item in a function (so it has the same markup as other menu items). The menu from appearance->menu has a dummy item. The function searches for that in the menu, and does a strreplace, replacing the dummy string/link with the constructed one. The function is hooked to the action wp_nav_menu_items.

See https://wpbeaches.com/add-menu-items-in-a-certain-place-with-wp_nav_menu_filter/ for a slightly simpler version of that.

You want to add a 2nd menu?

https://codex.wordpress.org/Function_Reference/register_nav_menus

You display a menu via https://developer.wordpress.org/reference/functions/wp_nav_menu/

I think you need to read through the section on menus and navigation here:
https://developer.wordpress.org/themes/