Hello,

You can put the icon name in “Icon name (from FontAwesome set)” field to display the icons before the menu name from Appearance >> Menus.

Screenshot: https://jmp.sh/PbWySjl

Result: https://jmp.sh/vKzpGRv

For the icons which you pointed out in your screenshot which only shown if menu have sub items, you can use below CSS code in global custom CSS code under theme settings:

.vertical-navigation .item-level-0.menu-item-has-children>a:after {
content:"\f2b9";
}

But it will change all the icons of that list.

Result: https://jmp.sh/fMN6C6F

I used “\f2b9” in my code just to set an example for you. You can use any other icon.

Best Regards.