I have never played with one, only studied the videos (e.g. https://youtu.be/o_TlE_U_X3c ) and documentation. It was a very radical UI. If you added programs, they just added new functionality to the existing editor. So, for instance, a spreadsheet module might mean that suddenly, when loaded, you can do operations like SUM{...} or MEAN{...} on a table of numbers in the editor.

I've long wished that someone would implement the Canon Cat UI as an Emacs module. It would seem to me that Emacs has all or most of the necessary functionality already.