I found an interesting article about the subvolumes and actually found the quote I had seen in getting my MX to control grub over Garuda in this document.
https://btrfs.readthedocs.io/en/latest/Subvolumes.html
" A freshly created filesystem is also a subvolume, called top-level , internally has an id 5."
I did not know what they were talking about on two different forums when they used the phrase,
So now I know where that came from. Reading this doc might shed some light on the parts we are not understanding and what makes Gentoo different from Fedora or other btrfs.
Sheila