MFIX learning ---how to know each file function?

hello everyone:
I have learn mfix for a while, and i want to learn the mfix deeper for the reason that i can implement certain functions by changing some source code, so i want to read the source code.
So, is there any learning materials to help me read and understand the logic of code? especially the organizational structure of the code and specific meaning of each file?of course the variable name description?
Thank you !!!

There is some documentation available at MFiX Documentation - NETL Multiphase Flow Science

Note that some of these files like the “Theory Guide” are quite old and may not be up to date with the MFiX code. However the “big picture” is still the same.

The best way to familiarize yourself with the MFiX source is just to dig in … if you have any specific questions, you can ask here.