Take PSG, for example: their stars are well-rested and don’t play excessive minutes, so they can press and run relentlessly in UCL matches. Meanwhile, we keep losing because Vini or Mbappe aren't chasing the ball. If they only played 1,400 minutes in La Liga instead of 2,400, we’d probably have a better chance of winning the UCL.
Here is the data for PSG player minutes in Ligue 1 :
Joao Neves 1,139'
Nuno Mendes 1,250'
Kvaratskhelia 1,323'
Vitinha 2,008'
Meanwhile our players in La Liga:
Tchouameni 2,315'
Mbappe 2,405'
Vinicius 2,484'
Valverde 2,567'
I think that should be a consideration. This also explains why, despite creating numerous chances, our players struggle to finish against Bayern, whereas PSG is clinical enough to score five goals against Bayern even with a much lower xG.