Some very cool books on this. Most of the population was centered around a few very dense southern fertile agricultural spots, #1 being the Mexico Valley based on farming corn. There was also some decent-sized populations in California and along the Mississippi river. Other than one large settlement called Cahokia near modern St Louis, there were never any major populations very far north, probably for the simple reason that it never suited their agricultural style. It doesn't take any genius to build a huge population, just organized agriculture and a good reliable water source. There are some really interesting primary accounts from early Spanish (well it was before Spain was a thing, but "Spanish") explorers and Priests who wrote about seeing massive settlements everywhere. On the Mississippi River they wrote that there were so many people the fires lit the sky at night and you couldn't even land on the shore. French explorers a century or two later just found empty land. Smallpox and even simple crowd diseases can wipe out entire civilizations of people who had zero natural immunity. If only they'd had Pfizer to save them (that last part is a joke guys, don't kill me like the clotshot does).