How did factories change the towns where they were located ?
In the 18th century in America, if a factory was opened in a town, the town probably had some type of geographic advantage, like access to water or rail or natural resources. The town also probably had some type of population base.
Depending on the factory, some of the changes could be: an increase of immigration as demand for labor rises, polution, crime, food diversity, more stores and an increase in infrastructure.
If the factory has staying power or longevity, such as steel or textiles, the change may be somewhat permanent, which would mean a small town could swell into a city within a generation as people move from the rural areas in order to have a higher paying job.