I retract part of my review, where I implied that the writers/directors handled time travel by not handling it. With Russo's confirmation, the time travel in this movie follows exactly what is explained (clearly): you can't change the past, any changes result in a new timeline. What isn't clear is what constitutes a change, but I can live with "significant", however vague that may still be.
The past is an alternate dimension/universe... an interactive recording if you will. Any "significant" changes will result in the continuation of this new dimension. Cap lived out his life in the "Shire" with alt-Peggy. They really left the door open for what happened over there. Since he didn't have to worry about timeline alterations since he already created the branch, did he get in more adventures? Where did the shield come from? Did we get 60's Captain America? Or did he just play it like we thought he would in the main timeline (keeping low profile with a new identity) How did Bucky seemingly "know"? Stay tuned