I would like to know how to:

  1. understand how FONTWORKS really works

  2. modify existing FONTWORKS designs [change clolors, fill patterns, etc.]

  3. create my own FONTWORKS designs

Where do I find the documentation that explains all that

Is FONTWORKS available in all LibreOffice [Writer, Calc, Impress, Draw, Math] elements

Documentation is here: click. See Getting Started with LibreOffice section Chapter 11, Page 14.

Also, as noted there:

Fontwork is available with each component of LibreOffice, but you will notice small differences in the way that each component displays it.

Fontwork is a special kind of a custom shape. You can use it everywhere, where you can use custom shapes. That is in Writer, Calc, Impress and Draw; but not in Math.

If you will make your own, totally new Fontwork shape, you can use a macro and the API, or save a document in flat format (e.g. fodg) and work with an editor on file source. If you really want that, tell us. Than we can give you some pointer.