The club have real time access to the GPS trackers on the players. They know if their running rate drops. They know if their sprint speed drops, or they start walking more, if they stop tracking back and pressing. Someone is sitting on the bench or in a box, watching this in real time for all 11 players and advising Pep and the assistants.
They know infinitely better than you if a player is tiring and needs to come off, or is still perfectly fit and just getting more match sharpness though being on the pitch.
So just stop fucking whining and pretending you know better than the club, and enjoy the football FFS.