Aren't you able to just copy an image from Preview or whatever, and go to the folder's 'Get Info' dialog and paste it into the icon area? I used to do that on MacOS 9 and macOS ~10.7
Yes, that work for you on your current system. But using a icns file as the source for setting an icon, the icon (quality) can scale across devices & system versions.
So ultimately it’s more of creating an actual portable icon file.
https://en.m.wikipedia.org/wiki/Apple_Icon_Image_format