“By conservative estimates, the population of the United states prior to European contact was greater than 12 million.”
where did they all go?
They mostly died from European diseases. Influenza and small pox being key players.
Per Jared Diamond’s book “Guns Germs, and Steel”.
The 12 million was for all of north and south America, not just the US.
A key question is why the Spanish king sent conquistadores to the new world, and why the Aztecs didn’t send conquistadores to Spain?
Why? Because the orientation of the Americas is north-south, and the orientation of the Eurasia is east west. Because of that, Eurasian plants and animals which were useful could spread east to west and be nearly uniformly useful all along the path. By contrast, plants and animals in the Americas that were useful had to cross climate zones where they were less useful, spreading more slowly.
Because of that Eurasian farm animals were more preferentially evolved, and the diseases evolved in those farm animals evolved along with them, also affecting the evolution of humans. When the Europeans arrived they got there with a lethal combination of disease, animals from which the disease moved to humans, and human keepers immune to the disease.
That faster development of farm animals lead to faster development of other technologies, such as guns and steel, but those were essentially defensive. The germs were the technology that could infect one native village after another, far in advance of the Spanish and later English colonists.