Photo Credit: Disney/Marvel Studios
Spoilers ahead for Avengers: Endgame.
In serving as the culmination to the Infinity Saga — that's the first 22 films in the Marvel Cinematic Universe — Avengers: Endgame had to tie up the character arcs of its original crew. For Captain America, that comes in the film's final moments, as we are shown that he lived a happy life with his love Peggy Carter after putting all six Infinity Stones back in time where they belonged. But old Cap also returns to our timeline to pass on his shield to Sam Wilson aka Falcon, which rightly raised a few eyebrows. Endgame co-director Joe Russo has now clarified that matter.
At an event in China — with translation provided by a Reddit user — Joe Russo said that “the time travel in [Avengers: Endgame] created an alternate reality. [Captain America] lived a completely different life in that world. We don't know how exactly his life turned out, but I'd like to believe he still helped many others when they were needed in that world.”
It naturally follows that there would then be two Captain Americas in that reality, and Russo confirmed that: “Yes. […] It's just like what Hulk said, what happened in the past has already happened. If you go back to the past, you simply create a new reality. The characters in this movie created a new timeline when they went back to the past, but it had no effect to the prime universe. What happened in the past 22 movies was still canon.”
That is both convenient and important for Marvel, so that the events of Endgame don't erase what audiences have seen in the past 21 films. It also meant that the characters simply couldn't go back in time and kill baby Thanos, as War Machine suggested, because the moment you pop back in the past, your present becomes your new past and your past becomes your future.
In an additional explanation, Russo added: “Both the Ancient One and Hulk were right. You can't change the future by simply going back to past. But it's possible to create a different alternate future. It's not a butterfly effect. Every decision you made in the past could potentially create a new timeline. For example, the old Cap at the end of the movie, he lived his married life in a different universe from the main one. He had to make another jump back to the main universe at the end to give the shield to Sam.”
Unfortunately, Russo didn't touch upon how exactly Cap managed to jump back to our timeline. Hopefully, we will get some clarity on that later.