[user104658]

Originally Posted by James

Spoiler:

Yeah, I think there are two Steves, with one still frozen until about 2011 in that alternate past.

They set the rule about time-travel when they go to the past they can only change history of that reality (or dimension), not the original one.

It seemed like they broke that rule at they end though when older Steve was sitting on the bench. He would have had to have found a way back to the original dimension to be able to be there. For instance he rebuilt the time-machine with the help of Hank Pym or Tony Stark from that dimension. Which is stretch and a lot of trouble just to be able to give Falcon the shield.

No; The Ancient One explained to Prof. Hulk that the infinity stones themselves are what rectify the paradox of time travel and keep the reality flowing "as one". Different timelines split off if an infinity stone is removed from the timeline which is why she doesn't want him to take the time stone. That would split reality at that moment and leave her current timeline defenseless. Prof. Hulk counters that returning the stones to their place in the timeline "as though they were never gone" should maintain the structure of the timeline, which is why it's so important for them to take the stones back where they belong.

Cap goes back and returns all of the stones, unifying the timeline, and is still in that same timeline.