The best answer should always come from Mike Engelhardt. Here is just my personal opinion. In the Qspice HELP section, under Acknowledgments, it is mentioned that Berkeley SPICE version 3F5 and the BSIM code from bsim.berkeley.edu have been extensively reworked for the Qspice project. Spice has a long history, and its work is built on a vast foundation.
For example, bipolor transistor model Mextram 504, NXP official document is 90 pages, and with few hundred device equation in that document. Possibly for anyone who are not work on device modeling (including me) has very difficult time in understand that even all formula are provided.
Mextram definition document : The Mextram Bipolar Transistor Model (nxp.com)
In this forum, Mike recommended a book multiple times, which is 2nd edition of Semiconductor Device Modeling with SPICE by Paolo Antognetti and Giuseppe Massobrio. This is a great reference and give me a lot of help when I preparing the device guideline in my Github especially the section of Diode, Transistor and MOSFET.
What are the best books on the SPICE simulator? - QSPICE - Qorvo Tech Forum
If Mike works on listing all device equation in help, possibly he won’t have time to work on Qspice coding anymore.
If you are not work in device modeling, possibly you just need to understand several critical parameters which help to slightly adjust the model to better fit the model into datasheet or measurement results. In addition, user likes @bordodynov in this forum has in-depth knowledge of device equation adjustment. So, possibly you can get help from this forum too.
I highly recommend the book recommended by Mike if you want to start your journey of understanding device equation.