The directors and the writers gave different answers.
At the end of Endgame, Steve returns the Stones, stays in the past with Peggy Carter, and turns up old to hand his shield to Sam Wilson. Did he make a new timeline or live in the main one? Even the people who made the film disagree.
Levels marked "Sourced facts" are checked against the sources listed at the bottom. Levels marked "Fan theory" are interpretation, not confirmed by the films or their makers.
After the final battle in Avengers: Endgame (2019), Steve Rogers takes the Infinity Stones and Mjolnir back to their proper times. He doesn't come back the way he left. Instead an elderly Steve is found nearby, and he passes his shield, and the name Captain America, to Sam Wilson. He has stayed in the past with Peggy Carter, married her, and lived a full life. The last scene shows the two of them dancing, the dance he promised her in Captain America: The First Avenger (2011).
Endgame's own rule (explained by Bruce Banner) is that changing the past creates an alternate reality. The directors, Joe and Anthony Russo, have said Steve did create a new timeline when he stayed with Peggy, and later found a way back to the main timeline as an old man. The writers, Christopher Markus and Stephen McFeely, have said Steve found a way to break the rules and lived with Peggy in the main timeline, which would mean two Steves existed at once; they said both were at Peggy's funeral in Captain America: Civil War (2016). Wikipedia's Steve Rogers article records both answers.
This part is fan debate. The directors' answer fits Endgame's stated time-travel rule but leaves open how Steve got back. The writers' answer explains the old man on the bench neatly but bends the rule the film set up. The film never shows how he returned, so there's no on-screen evidence that settles it. Both readings are made by people who worked on the film, so neither is 'wrong'; it's fair to say the question is genuinely unresolved.