You can create entire new folders and populate those folders with the shortcut icons to the applications you want (and with sub-folders). Just look at the existing icon Properties of the applications to find out what to put into the dialog box when you're creating a new shortcut.
If you want to move an icon or a folder up or down, select it, then click the Move Up or Move Down buttons at the right-hand side of the application... although, my Zorin menu is alphabetically sorted, and I can't find any way to sort it differently or rearrange it. So moving the icons and folders in the Main Menu application may have no effect upon the Zorin menu sorting.
Thanks! Yes, it helped a lot. The issue I had was that you need to assign an icon before it will accept the new menu or menu-item. Having to do that makes sense in a way, but also not.
(When you're adding/changing something, click on the icon to choose a new icon. A lot of icons in .cache/gnome-software/icons and in apps' own folders.)