I genuinely believe it is a combination of money and superb coaching. The latter acts as a catalyst with the former.
Coaching, club mentality (see Madrid that somehow fudging change into a different side in knockout rounds) and lots of luck (draw, injuries, etc.)
The coaching one is interesting with the Pep example, won CL twice with a once in a generation team, then has basically brick the bed for a decade with 3 of the top teams in Europe with almost unlimited budget ..