What about his upbringing gave him the ability to recall pages from books he’d read decades later?

Nothing. They put effort into his education because of his brilliancy, and your standard Jewish-American household could do the same thing.

They gave him multiple private teachers by the age of 6 - at that time people would normally not even be thought arithmetic.

People can recall pages from books if they put the effort, and regular people routinely do - an average person can memorize thousands of pages. Again it's that he could do it faster that's exceptional.

> Neumann was supremely intelligent, beyond the conception of regular people

And they would be right. This sort of genius is not only nurture and hard work, but nature as well. Like no matter what I do in my life, I will never even approach that level.

Of course he is still human, and someone will born (rarely though) that will achieve similar feats, but let’s be honest and just say that what he did is well beyond the reach of the average people.

You may not, but hundreds if not thousands of people were able to push his work farther than he could. It's not beyond the ability of a regular person with enough work to understand what he understood. Really the biggest difference is speed.

Understanding something is not the same as being able to recreate it from scratch.

I can follow a math example’s solution, but that doesn’t necessarily mean I could solve it alone.