Max Verstappen drove for Red Bull Racing when he won his first Formula One world title in 2021.
Verstappen entered the 2021 season as Red Bull's lead driver, partnered by Sergio Pérez. He won 10 of the season's 22 Grands Prix and fought Mercedes driver Lewis Hamilton for the championship throughout the year. Their points were tied before the final race in Abu Dhabi.
At that finale, Verstappen qualified on pole but fell behind Hamilton during the race. A late safety-car restart created one final racing lap, and Verstappen passed Hamilton to secure both the victory and his maiden Drivers' Championship. Red Bull Racing used Honda power that season, making Verstappen the first Dutch Formula One world champion.
This question concerns the drivers' title, not the constructors' title. Mercedes won the 2021 Constructors' Championship, while Red Bull Racing finished second in that team standings contest. That distinction is why the answer is the team Verstappen drove for—Red Bull Racing—even though Mercedes won the other championship.