Whoever is in charge of fitness and health science there is doing an incredible job. Juve conserves energy for half the season and cant do 90 min of high tempo football
This conserving energy thing doesn't work in todays football and should be a thing of the past. Mostly you have to play full throttle all season and you'll be rewarded the most.
I also think thats why the likes of Liverpool, Real, Bayern and to some extend City etc have been so succesful in recent years. These teams are fitness monsters and can run for days.
Can't speak for the other clubs, but in the Amazon documentary about Bayern you'll see a lot of work and details are put into the physical conditions of the players, stamina and strength especially.
I imagine it's the same under Klopp and in Real Madrid.