Bayern Munich played with 4-2-3-1 line up. Leipzig played with 3-4-3 in the first half and second half’s first few minutes. Then they turned to a wide 4-1-2-1-2 line up. But I will explain their problems in 3-4-3 line up.
When Leipzig were in set defence, they were still like 3-4-3. Forward players took position centrally between Bayern Munich’s defense and midfield lines. They weren’t aggressive but also didn’t let defenders to dribble forward comfortably. But they covered only central spaces and meeting full backs was Leipzig wing backs’ task.
When both wing backs closed to midfield to meet Bayern Munich full backs, this time 3 stoppers matched with 3 Bayern forwards 1 by 1. And when Bayern Munich’s offensive midfielder Müller made runs to between Leipzig defense and midfield lines, Bayern just basically had 4 vs 3 numerical advantage. And because they had this numerical advantage in forward, they could go to score goal directly. Because basically, when one player is already free in forward, you can pass the ball to him directly without a lot of passes and effort. And you can raise the tempo because you can change the ball’s location directly from defense to forward and quickly in less time.
This position is before Bayern Munich’s second goal. As you can see, 3 Leipzig stoppers match with 3 Bayern Munich forwards. Then Müller runs to defense line.
Müller takes the ball, passes to Musiala (who has the ball now) and keeps running. Now Bayern Munich forwards have numerical advantage and as you can see, Leipzig’s left stopper Halstenberg has to cover two forward players by himself. And he can’t cover both at the same time (because he is a human) and in the attack, Mane will score goal to nearly empty goalpost.
In my opinion, meeting Bayern Munich defenders with 3 players directly was not necessary because Leipzig didn’t press high aggressively. I mean if they would press aggressively, 3 forward players and 2 wing backs would be so effective against Bayern’s 4 defenders. 4 defenders couldn’t pass the ball to each other because these 5 players of Leipzig could press them quickly and they had to play more directly without thinking much enough and it could bring some advantages for Leipzig. But when you don’t press high, use 3 forwards to match with opponent defenders and also you try to cover wings nearly all the time with wing backs, this time central and closer to your own goalpost spaces would be more for opponent. And because you don’t press high, opponent players can pass the ball to these spaces comfortably.