What does std::mutex prevent threads from modifying?

Question

What part of memory gets locked by mutex when .lock() or .try_lock(), is it just the function or is it the whole program that gets locked?

Answer 1

Nothing is locked except the mutex. Everything else continues running (until it tries to lock an already locked mutex that is). The mutex is only there so that two threads cannot run the code between a mutex lock and a mutex unlock at the same time.