Complex sentence: The Great Depression had a significant impact on the Brown Company, which was the major employer in Berlin, then with a population of about 20,000 people, most of whom either worked for the company or provided services to the families of company employees.
Split into simpler sentences: The Great Depression had a significant impact on the Brown Company. Berlin at that time had a population of about 20,000 people, most of whom either worked for the company or provided services to the families of company employees.