By far the biggest controversy about Avengers: Endgame since its release has been Captain America’s ending. Combined with confusion about how time travel works in the movie, fans have been hatching all sorts of nitpick theories based on how Back to the Future deals with time travel and not how Avengers: Endgame does. The theories have spiraled out of control with people not quite understanding how Cap’s ending worked out, but now there’s an official explanation.
Joe Russo gave an interview with a Chinese site, which u/gianben123 on Reddit translated, and he very clearly explains what happened with Cap, the timeline, and how he got back on that bench at the end of the movie:
Q: Did Captain America’s action at the end affect the timeline? Does that mean there was a time where two CA existed in a same universe?
A: To me, CA’s action in the end wasn’t the fact he wanted to change anything, it’s more like me has made a choice. He chose to go back to past and lived with the one he loved for the rest of his life. The time travel in this movie created an alternate reality. He 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. Yes, there were two CA in that reality, it’s just like what Hulk said, what happened in the past has already happened. If you go back to past, you simply created a new reality. The characters in this movie created 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.
Q: EG’s plot, is it a parallel universe or a closed time loop?
A: Nope, not a time loop. Both 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 butterfly effect. Every decision you made in the past could potentially create a new timeline. For example, the old Cap at the end 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.
So there you have it, that appears to the the official explanation for Cap’s story in Avengers: Endgame. He created a new timeline when he went back into the past to live with Peggy, what happened in the MCU is still canon, and he had to make one more time jump back to give Sam the shield. This should (hopefully) clear up all the confusion about what happened at the end of the movie.