The figures given are annual averages for total labor force in city metropolitan areas and come from the Local Area Unemployment Statistics from the Bureau of Labor Statistics. They define the civilian labor force as “all persons in the civilian noninstitutional population ages 16 and older classified as either employed or unemployed (https://www.bls.gov/lau/laufaq.htm#Q03).”
Data was not available for Chula Vista, Irvine, Santa Ana, and Plano. Additionally, for many cities the data given actually comprises several contiguous municipalities which make up the metropolitan area. A list of the metropolitan areas for which the BLS collects data can be found here in this report.
In some cases the same number is given for two areas because they are considered as one metropolitan area by the BLS, as in, for example, Mesa-Phoenix, Minneapolis-St. Paul, NYC-Newark-Jersey City, Los Angeles-Long Beach-Anaheim, Dallas-Fort Worth -Arlington, San Francisco-Oakland-Hayward, Denver - Aurora and Las Vegas-Henderson.
To obtain the data from the latest link above, select all the states, then “Metropolitan areas,” then all the Metropolitan Areas (alternatively only select the cities for which you want data), then “Labor Force” and then check “Not Seasonally Adjusted” and click “Retrieve Data.” At the top of the page you can adjust the year range to your specifications and include annual averages and then click go to update.