They always have their own way of making them unbeatable until now, they never give up in every match, and actually this is not the first time they have succeeded in refusing to lose at the end of the match, because they have also done the same thing before.
Xabi Alonso's team already has high motivation and I think that is implemented by Xabi himself. Because when he was still active as a professional footballer, he was also a player who had a high mentality, especially when playing for Real Madrid. So it's not surprising that he knows what he has to do to make his squad have a good mentality.
Alonso managed to extend their unbeaten record this season to 45 games without defeat in all events, I think Dortmund will be able to give Leverkusen their first defeat after they went ahead in the 81st minute, but as you said Leverkusen were able to recover even when the game only had 1 minute left, I think this season Leverkusen are not allowed to lose because it seems like every time defeat is in front of their eyes, They were able to at least level the score to avoid defeat even with only a few minutes left, what Leverkusen have done this season looks like a miracle that probably no other team will be able to do.
The experience and mentality possessed by Alonso during his time as a player of course he managed to transmit to the Leverkusen squad this season, what you said is true, Alonso's experience I think is one of the main assets for Alonso to build the Leverkusen squad so far, besides that I think Alonso is quite capable in seeing every weakness of his opposing team by applying a strategy that is indeed able to destroy the opponent's defense and Alonso has played in three leagues What is different, of course, has made him understand how the character of the club and the opponents that Leverkusen will face.