How to qualify for the home and away two rounds of European Football Champions Cup knockout?

Take AC Milan's 4-2 elimination of Bayern in two rounds as an example.

Milan drew 2-2 at home by Bayern. Bayern scored two valuable away goals. One away goal equals two goals.

After arriving at Bayern's home, Milan must beat their opponents or draw more than 3-3 to qualify. If the game is 0-0 or 1 right 1, Milan will be eliminated because of fewer away goals.

If Bayern's home score is 2-2, it will be overtime, and if it is still tied, it will be a penalty shoot-out.