Between 2010 and 2020, the US population rose from 309 million to 331 million. Population trends vary by county. Over half of the 3,143 US counties lost population during the decade. Counties losing people are often rural, while most metro countries gained residents. Over 86 percent of US residents were in metro counties in 2020.

Maricopa county (Phoenix) gained the most residents over the decade, almost 754,000, followed by Harris county (Houston) with over 630,000. Five of the 10 counties adding the most people over the past decade were in Texas. Midwestern counties lost the most residents, led by the net loss of over 90,000 residents in Cook county (Chicago) and almost 75,000 from Wayne county (Detroit).

Source: https://www.census.gov/library/stories/2021/08/more-than-half-of-united-states-counties-were-smaller-in-2020-than-in-2010.html