The Boston Celtics have been a truly elite defense in the playoffs. The injuries have killed them, but Brad Stevens has found a way to turn every single remaining player into some sort of asset on defense, and that has allowed them to surprise a lot of people in the playoffs.
The Celtics always play defense as a unit, and even if they have their matchups, everyone is expected to play multiple different roles. The Celtics have the best defenisve communication in the league, and everything is about versatility.
The Celtics have the kind of players that are always versatile, and the key to their success is having the communication to make sure everyone knows where they need to be at any given moment, and none of those positions being exploitable.