When an extension is installed via .rbz script, where do the files get placed on a Mac? I cannot for my life find them. Specifically I am wondering what the file path to said icons is once installed.
They are placed in a subfolder within your Library folder. Finder normally hides Library, and that is probably why you haven’t been able to find them. You can use Finder->Go->Go To Folder and copy-paste the following in the dialog prompt:
~/Library/Application Support/SketchUp 2022/SketchUp/Plugins
OMG Thank You! it’s the little things in life, like a file location after an hour of looking. 
As an alternative, if you press and hold the Option key and then open the Go Menu, the Library item will be listed in the menu.
You can also type Shift Command L to open the Library folder.