What did thanos had to do in order to win from Infinity war to Endgame in Marvel universe?

The theory of old Captain America's wife :

When Cap traveled back in time to return the stones, he got Nat back in exchange for the soul stone and then Nat helped him return all of the remaining stones and she helped him fix all of the stuff that they messed up when they collected the stones... he didn't marry Peggy because that will change the future, and marrying Peggy will prevent her from marrying the husband that she married when Steve froze in ice, so he just stopped by to dance with her.

Then when they were ready to go home, they only had 1 pym particle left, so they promised to stay and only return if one of them dies. Years passed and Nat and Steve got married, lived life together until Nat died and that's when Cap returned to inform everyone that everything was okay.

But when Sam asked who the girl Steve married, he refused to tell him because it was actually Natasha.”

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