Complex sentence: In the end Oskar finds the man the key belongs to, but he doesn't want to know what is inside the bank box, because it doesn't have anything to do with his father.
Split into simpler sentences: In the end Oskar finds the man the key belongs to, and what it opens-a bank box. But he doesn't want to know what is inside the bank box, because as it turns out, it doesn't have anything to do with his father.