Zorin OS, including Zorin OS Ultimate, is open source. If you wanted to and had the time and expertise, you could build a clone of Zorin OS Ultimate on your own.
As I understand it, Zorin OS is what the Zorin Group does full time. If they did not do it full time, we could only imagine the gaps between release dates and the silence that would cause. Choosing Ultimate supports this development, but Ultimate is not exclusive. It is a want, not a need.
Finding all the documentation: as you point out, is not so easy. Rather than search outside, I searched within by dissecting Zorin OS.
I have no intention of making a Zorin OS clone, don't get me wrong. But I would like to know how, just in case. Plus, it shows how very little I know and how much I have to learn. 
You are able to search the public packages included in Zorin:
As well as backtrace those to other open source, like GSConnect and Arc Menu.