There were no "full towns" built. There were different set ups for specific effects tests, e.g. Shot Teapot APPLE-2, which is what things like Indiana Jones was based on. The idea was to find out the effects of nuclear weapons on different types of structures, substances, etc. As for conclusions, they had lots of little technical conclusions about what kinds of civil defense mechanisms might work under different conditions, what kinds of houses are more likely to burn under what conditions, what kinds of foods would be edible after enduring a radiation flux. (I wrote an article on the latter for food magazine a little while back.

There were severeal other effects tests over the years, both in the USA and other countries, but most were small-scale. To me one of the most interesting one was the setup used for Shot Upshot-Knothole GRABLE, where they built a tiny forest to see whether the trees would ignite or not. It took a lot of work to set up these kinds of shots.