I have found these
https://github.com/ggaughan/ThinkSQL
https://sourceforge.net/projects/tpflashfiler/
Have you actually read the description that goes with ThinkSQL?
The source code is linear per se, but while writing it, it was an organic thing, generating great interwoven trees in the computer memory and in my head that were many-layered and that were modified and traversed, and impacted on a dynamic multi-versioned data store alongside many other threads, causing and needing deep psychological flow. The code comments are released as-is, and are often streams-of-consciousness.
A number of other people have given you some good suggestions and to those I'd add that you might find it worthwhile looking for an early version of PostgreSQL, i.e. v6 or older, which basically started off as a teaching/research exercise so at least in those days was well documented.
Look, I'm sorry to be brutal but software is /complex/, and the more man-years have gone into a project the more complex it's likely to be. Alternatively, if a project was left on a hillside at birth it's likely to be valueless, and a waste of time in terms of both utility and didactic potential.
If you think that, as somebody with admittedly-limited experience, your chances of getting to grips with a project are dominated by your familiarity with the language it's written in then you're fooling nobody but yourself: these days you /need/ at least limited familiarity with C/C++ (and, regrettably, with Python and Javascript) even if you have no intention of writing anything in them.
MarkMLl