How is the presence of Captain America explained? [duplicate]

It's the young Steve from the present that went to the past to live with Peggy. When he goes into the past to return all the Infinity Stones and Mjolnir to their original timeline, he makes a choice there and decides not to come back to present. Later, that Steve appears as an old man.

As director Joe Russo explains,

"It's not a time loop," according to Joe Russo in a recent Q&A. "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."

From a reddit thread with the translation of original article,

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.

In conclusion, it was an alternate timeline created by the choice young Rogers made.