Hayley Atwell first played Peggy Carter in "Captain America: The First Avenger." She even had her own spinoff show, "Agent Carter," that aired on ABC from 2015 to 2016 (and it was an absolute delight). The MCU retconned her ending on that show, though, in "Avengers: Endgame," when Steve Rogers aka Captain America (Chris Evans) travelled back in time to live his life with her instead.
In this universe in "Doctor Strange the Multiverse of Madness", though, it's Carter who was given the super soldier serum, turning her into Captain Carter. A similar story was told in Marvel's "What If...?" on Disney+, though it's not clear that this Carter would have the exact same origin. We probably won't see Atwell's Captain Carter again, but there's always a chance.