If you are trying to understand a large body of source code, and documentation to guide you is scarce, I find that it helps to sketch out diagrams of how all the pieces relate to each other. Likely a good way to get started is to get yourself a copy of the code and work out how to build the application executable from source. Tools that help you probe for a given function or variable (e.g. cscope for C/and its relatives) can be super helpful. Worst case scenario, grep is your friend. I haven't tried it, but I see there's a pyscope that says it can build a cscope index for accessing a body of Python code.
Do the world a favor and generate a tour guide document to help people who find themselves wanting to understand the same body of source in the future. Fine details will change, but the overall structure of the application likely will be stable for long stretches of time.
sloccount is another way to summarize code by size and complexity. May help you zero in on the most gnarly pieces of the application (or depending on your point of view, may tell you when to avert your eyes).
I'm old fashioned enough that when I'm digging in detail into a specific complicated module, I like to have a pretty-printed hard copy of the source code. I remember one TCL program I was wrestling with. The in-house TCL pretty-printer program choked on some of the code. Turned out there were some coding irregularities in the modules that made the pretty printer choke, so on the one hand I worked with the maintainer of the pretty-printer program to make it more robust in the face of errors in TCL source files, but on the other hand, I cleaned up those flagged modules and looked into the test data to understand why those errors hadn't already been known and corrected. Once you have a hard copy, then a 3-ring binder, paper notebook dividers, a highlighter, a pen and a stash of sticky-notes to bookmark important pages can all help in your expedition.
If your target is still undergoing rapid change, then my suggestion of getting a hardcopy may be unwise as the hardcopy print out won't automatically reflect the latest in changes that are going on even as you are trying to get your footing.
Happy digging.