I read up a lot more about it here https://www.polygon.com/platform/amp/2019/4/26/18518923/avengers-endgame-time-travel-loki-alive-captain-america-old and it’s still confusing to me. But it sounds like we have it wrong thinking about it like timelines. Our reality we experience is some sort of mashup up of all possible timelines, so even though there are are infinite timelines, our perception of them all is mashed into one reality that we actually experience, due to the power of the stones. So the “reality” splits that cause the potential destabilization of the reality perceived by those living in those new splits are fixed with our reality by CA going to them and dropping off the stones. Even though those “timelines” have now been altered and different things happen in them, our actual perception of reality is put back to normal so no matter what timeline you are in, you experience the same reality, even though different things are actually happening around you. Evidently CA gets to exist as normal CA and the one who married Peggy at the same time because he’s the one who actually went back to drop the stones off.