What city was part of the Manhattan project had 75000 citizens and wasn't named until 1949?

The city you are referring to is Los Alamos, New Mexico. It was established as a secret site for the Manhattan Project during World War II, where scientists worked on developing the atomic bomb. Although it had a population of about 75,000 at its peak, it was not officially named Los Alamos until 1949, after the project concluded. The city's history is closely tied to its role in nuclear research and development.