[hackermailman]

Universities all have these kinds of intermediate courses: https://www.cs.cmu.edu/~ckaestne/17313/2018/#schedule is an example of general software architecture like how to identify requirements. Notice all the books on architecture and design being referenced as well. There's startup architecture courses floating around too: https://cmu-17-356.github.io/

For exploring some huge codebase where there is no design documentation one way is just to use a fancy ide and sabotage something to see what it breaks in your ide, then you can trace what code is relying on. Some open source projects have books written about their architecture, you could also start with those then everything you see there is often similar in undocumented projects in my experience, in terms of organization of code like where is the directory containing all the core functionality, where are the feature addons.

[erikpl]

Do you have more information about that startups course? Looks very promising by the looks of it, but the resources they provide in the schedule section are a bit sparse :p