To take another shot at defining what is meant by the term ROOT in Unix based systems, and MacOS is a Unix based system. Perhaps these umbrellas will help you see the relationship. Using Unix notation, the pathname of your documents folder would be /Users/UserName/Documents. Because ~/ is Unix shorthand for /Users/UserName/ that same pathname can be abbreviated as ~/Documents.

For all practical purposes, the drive root will aways be Macintosh HD (assuming that is the name of the currently booted Volume Group).

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein