I don't think it describes the same thing this post does (how to create a tree from Emacs' standard linear undo on the fly, in order not to store the actual tree). But both systems - linear and tree - are described extensively in relevant documentation.
Undo-tree mode has an extensive description of how it works, along with how traditional and Emacs-builtin undo systems work, inside its readme. You can view it from Emacs' package manager, or on the web here: https://elpa.gnu.org/packages/undo-tree.html, or at the top of undo-tree.el source file (easiest way is to get help on any function/variable from it, and jump to source from help screen).
