I fail to understand why this problem persists. I try to research a problem in hopes of finding a solution before I ask for help. What I found is that a LOT of users have this same problem.
I just purchased Painter 2021 which included several different brush categories. After unpacking the brushes they were enclosed in a folder. I was expecting this icon:
How do I install brushes that are in a folder?
By accident, I learned how to install the included brushes. I use a Mac and this is my procedure to install the Fine Art brushes:
Your Mac may have files in different locations than mine, but the process is the same.
For what it's worth, Skip Allen's page on brush installation and my post on the Graphic Design Stack Exchange on the two folders where brushes can be found (admittedly, PC-centric).
And incidentally, I love Skip Allen's brushes... thank you for the reminder to port my custom library from 2019 to 2020.