Thanos wielded all six Infinity Stones and snapped his fingers. Benji was one of the trillion that dusted, including Bucky Barnes, Peter Parker, Sam Wilson and so many other members of the Avengers.

All the remaining people had to do was assemble others that were absent during the Battle of Wakanda, which included Tony Stark, Nebula, Clint Barton, Scott Lang, etc. and reverse the effects of the snap.

Simple right?