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).