This was originally a Twitter thread, posted on 22 APR 02019.
Changes from the Twitter version: Lightly edited or re-expanded from the original text, linked to sources, and notes.
Addendum, October 2022, after the release of Black Widow filled in significant aspects about Natasha, and Falcon and the Winter Soldier filled in some details about Sam and Bucky. I have also updated links that experienced bit rot to their archive.org instance.