Tony Stark uses the Infinity Stones at the end of Avengers: Endgame, wielding them through a makeshift nano-powered gauntlet built into his armor. During the film’s final battle, the alternate 2014 version of Thanos takes the Nano Gauntlet and prepares to erase the Avengers. Tony secretly transfers the six Stones from the gauntlet onto his own armor.
Tony then snaps his fingers and uses the Stones to destroy Thanos, his army, and the alien forces attacking Earth. The Stones’ combined energy fatally damages his body, and he dies shortly afterward, making the moment the culmination of his character arc from self-centered weapons maker to self-sacrificing hero.
A common mix-up is Bruce Banner’s earlier use of the Nano Gauntlet. Banner uses it to reverse the Blip and restore the lives erased by Thanos. Thor also briefly attempts to kill Thanos with Stormbreaker, but Tony is the character who uses the Stones in the film’s final confrontation.