We can get to how Manchester United won in a moment. First off, here's why they won. They won because of the away goals rule. Not that they needed it to go through. The aggregate score is 2-1. Yet Manchester United won what was a very tight encounter, because they scored in Rostov, which meant Rostov had to score at Old Trafford to have any possibility of progressing.
So they couldn't throw a blanket of defensively massed yellow shirts over the game indefinitely, as they did in the first half. Eventually, they had to start playing, they had to try to get that away goal, had to make the match open.