Personally I've never needed to unpick a full blown standalone app like Chromium before, I'll be using repo code or dropped into some hulking spaghetti massive legacy app that needs a bugfix, but there's no-one left at the company that has any idea how it works and the documentation is _lacking_.. but if this helps then good, otherwise, oops, sorry for the pollute.
Breakpoint methodology wins for me simple and true.
I imagine it like pathfinding the minotaurs maze, you stand at the last place you recognise and can get back to (if that's literally the first active line then that's fine), and put something there (breakpoint, print statement, log line), run it and check you still know where you are.
Then put another down as far forward from that point that you can 'see', if that's literally one operation step then fine, spin it and check. Breakpoints are easily put down and just as easily cleared back up again. Keep only as many as you need to see which branch you are on.
Pretty soon you'll have run the damn thing so many of times you'll know it's bootstrapping and foibles and they will be second nature. You'll start seeing how it's generally laid out, you'll know where the main start up branchings are. When they leap into async or hidden 'rooms', log lines are perfect.
When the engine of it starts moving in your head, then is the time to start throwing breakpoints, prints or log lines in places that originally were completely unknown but now you have a feeling for. It's at this point you'll be bloody close to where you want to be.
Oh and do future you a favour, at least jot down something as you're going through this.
I find that this initial torchlighting is remarkably gratifying but if you don't make notes in six months time it'll be completely gone, and you'll have to do between a quarter
to a half of this all over again before the lights start lighting on and you remember how it's laid out.