Where is the official documentation for the URL parameters/arguments?
There are things like /preview, /copy, /export,/htmlpresent #headings, ?something question mark, that I stumbled on tips and tricks articles on the web.
Where is the Official URL API that explains ALL of them?
I'm not sure if this is what you are looking for, but this references contains all information and guidelines related to Google Slides API: https://developers.google.com/slides
https://developers.google.com/slides/how-tos/overview
https://developers.google.com/slides/reference/rest
You could also add more details on what you are trying to look for so I assist you better