How do Icon? files work and how to create your own
The purpose of Icon? is to display the icon when you click "Get Info" or Command + i. You can see it in the top left side of "Get Info" window.

There are many apps and help documents with which you can create icons. Check out few of them listed below.


Apple Support
Image2icon
How to Make Mac Icons
IconJar


There are tons beautiful Mac icons available online. Google is your friend.