These things are knowable, but unless you've spent some time studying it intensely, it's certain that you only know a fraction of the places where the program is written.
If it's helpful, programmers should imagine that it's written in C. At a glance you can tell what something's doing, but once you study it you can find UB all over the place and suddenly it's hard to say what the right answer is until you know the intricacies of the compiler and the target platform. You can't really determine the exact behavior without all that information that lives outside the code. Now, once you have all that, you can surely reason through it all. But how many people actually know all that, or even realize which parts they don't know?
"This is pretty straightforward" is a sure sign of someone who doesn't actually understand it well.
