How Rotation in the game works?

I am the author of the simpleplanes stl to xml converter written on matlab, and I am writing a new stl to xml model generator for simplerockets2, this time in C#. The majority of the codes are finished. However I encountered a problem: I do not know how rotation in the game works.
Sadly the computer I worked on the Simpleplanes xml converter is not with me atm, and it is literally tens of thousands of miles away so I cannot refer my old work.
Please, I really need help to understand how parts rotate in the game, what x y z axis are.
Thank yall.
Here's the stuff generated with current code: it is supposed to be a sphere: https://imgur.com/a/AB4Qboe