Financially Bayern is rich and that part is true but other German teams are not exactly poor neither. I mean look at Dortmund, they sold a few players in the last few seasons that would dwarf what Bayern sold, and yet even with that they failed to build a great team with all that money. This is a proof that it's beyond just transfers and buying players from bundesliga or anything else.

It is about buying players who are good for cheap. Look at how cheap they got Mane for, many teams pay huge sums for players that talented, and Bayern paid peanuts for him. Losing Lewandowski was a big hit for them, but they recovered very easily and got a new star.