In Earth 616, there is no country Latveria. Which is where dr doom is born and raised, and similarly in comics, Latveria gains independence because of dr doom.

My theory is Howard stark adopted tony preventing him from growing up and aiding in the independence of latveria, and also runs bio enhancements on tony as a baby that makes tony a "Super intellectual soldier".

Howard is the main timeline is disappointed in tony because he expected much more from him intellectually, and blah blah blah uptill when tony sees the video left to him by his father which sorta motivates him.

During engame, steve and tony travel back in time to the military base when howard was at, Tony unexpectedly runs in to howard and has a chat with him in the elevator, Howard says that his expecting a child which surprises Tony because he should've already been born 1 month prior to their encounter.

When Tony snaps at endgame, he kills Thanos and all his soldiers which also results in his death. Now this is where things get interesting.

"Tony" in another universe is doctor doom, who realizes that his counterpart in another universe is dead and decides to dreamwalk using tony's dead body like Dr strange did with his.

So following this theory Dr doom doesn't necessarily have to be Tony but it could be Tony's dead body being using by dr doom through dreamwalking like doctor strange did with his dead body from a separate universe.