Making Nav Menus ‘filterable”?

Hi and thank you for your time and patience.

DESCRIPTION:
I want to add icons to a nav menu (primary). I have learned that the menu must be “filterable”. I am learning /teaching myself to create themes. Working on localhost.

I have two themes that I am working with. One I created and is working pretty well. The other is “wp_bootstrap_4” which seems to no longer be supported by it’s author and WP, although it works pretty well out of the box as is. Both of these themes use the “new bootstrap navwalker”. Neither is able to display icons in the menus (as is a native bootstrap 4 capability) when used as is in a WP Theme.

Although I prefer to code, rather than use plugins for everything, I found that the theme that I built is able to show the icons in the menus with the help of a plugin called “Menu Icons by ThemeIsle”. The wp bootstrap 4 theme still does not show menu icons with that plugin. ThemeIsle notes tell me a menu must be “filterable”. I’ve done a bit of searching however have found no clear answers or process to make a menu “filterable”.

QUESTION:

What makes a menu filterable to enable the ability to add icons or even perhaps other items or functionality in a menu? How to do it simply so that a plugin could be used, AND how to do it (write the code or use existing snippets) without the use of a plugin? I’m not making heads or tails of the references in the Codex.

Thanks again!