You can create custom folder icons by using only the software that comes with your Mac. By taking the existing folder icon graphic and bringing it into Preview, you can tint the color and then add text, emoji or shapes to create a useful custom folder icon. Changing folder icons can be a little tricky and buggy, however. You can also use graphics apps to create more highly-customized icons and download icons online to use.
There are three add-on tutorials that I made that go along with this video. They are all at YouTube:
How To Create Custom Folder Icons With Acorn
How To Create Custom Folder Icons With Pixelmator Pro
How To Find and Use Custom Folder Icons Online