I was also confused how this could work in holdem. After some googling it appears that "counting" in poker only refers to certain stud games where hands ahead of you are exposed.
In holdem the only (?) additional info you could have over a competitor is if someone in early position folded and revealed their cards (obviously uncommon). That's info that the small blind wouldn't have had, and could be very useful if you're in late position, especially on the button. But imo that's not really "counting", just gaining info to craft your betting strategy.
Decent answer here: