Biggest strength of Leverkusen is from team working because they don't have star players. Some of their players will become star players but for now, they are not. Together by playing and supporting each other, they are making a strong team and seriously challenging Bayern Munich for Bundesliga title.
After the arrival of Xabi, Leverkusen turned into a serious challenger for Bayern Munich, just by relying on ordinary players but Leverkusen right now have proven that they cannot be underestimated, so far we have known Leverkusen only as a mid-table team but now Leverkusen has turned into a giant team in the Bundesliga, the surprise given by Leverkusen makes Bayern Munich now have to worked very hard to be able to win the Bundesliga trophy and they could even lose this season trophy if they are not able to improve their playing performance, If we compare the composition in terms of player prices for Bayern and Leverkusen, it is certainly very different, but as you said, Leverkusen is actually more optimal without any star players in their squad, the even quality of the players is a strength for Leverkusen at the moment and I think they are more united without the selfishness of the star players.
Xabi Alonso has shaped Leverkusen since last season so it is only natural that the peak of their performance can be seen and what they have shown this season is the result of the process they have gone through.
Xabi Alonso's influence is quite rapid and no one can deny that he is a coach who has quite good qualities.
To build a strong team you don't always have to have expensive or quality players, good chemistry and equality of quality in all lines also have a big influence and that has been proven by Leverkusen this season.