If most people in the U.S. died on 9-11-01 on the 1st timeline what would the U.S. be like on that timeline in this time period?.

There is no civilization at all. The people that survived could not breed healthy children so whatever people survived were the last educated people in america. in the 4th world that was destroyed on 9-11-01 on the 1st timeline, mostly you and I and everyone we know is dead with few exceptions being people not downwind of a nuclear blast. IN the next year after that blasts took out all major cities people fought over food and water and the ability to make modern day technology was lost due to emps frying everything nationwide so it didn't work any more that was electronic in nature.

So, no cars, no trucks, no cell phones, no computers, no gasoline reciprocating engines no water pumps, sewage pumps, gas lines, no really anything left of our present infrastructure. So, it was might makes right which was sort of like Mad Max movies. Something like that for the next 20 to 50 years. But soon, all the normal people died and all that were left were people not capable of learning to read or write because of radiation altering their genomes (genetics) so we quickly devolved past caveman down to a lower level than that from radiation poisoning.

So, within 50 to 100 years there were no educated intelligent humans left in the U.S., Russia, China, Africa, South America and the only completely untouched area by nuclear weapons was Switzerland because Switzerland is neutral as a country.

This allowed (eventually) because people there didn't have radiation poisoning as bad as the rest of places on earth to eventually evolve transportation beyond the wheel and because of books and libraries that were not nuked out of existence there, people still could somewhat read German, English, French, Spanish and a few other languages because of books and people teaching each other.

However, technology as we now know it was lost for several thousands of years into legends. Because if you don't have gasoline engines or workable electrical generators or workable electric lights because of EMPS you cannot easily create the infrastructure to replace all this overnight or ever if enough people who knew how to do this are now dead.