That's at the deepest level. I'd say you can stop one level up: knowing which function serve which purpose, which functions call which functions, which objects

That's at the deepest level. I'd say you can stop one level up: knowing which function serve which purpose, which functions call which functions, which objects maintain which state etc. If you can trace any input/request from the event handler all the way to the database and back, then that's a very good start.