Complex sentence: Menchell began most of his comedic standup in the Borscht Belt, what is now known as the Catskills, a destination of mostly New York City Jews from the 1920s to the 1970s.
Split into simpler sentences: Menchell began most of his comedic standup at Borscht Belt or what is now known as the Catskills. The Catskills was a destination of mostly New York City Jews from the 1920s to the 1970s.