Census

Wikipedia summary of population: https://en.wikipedia.org/wiki/1920_United_States_Census

Occupations: https://www.census.gov/library/publications/1923/dec/vol-04-occupations.html

Page 15 of this document shows percent of people employed in each geographic location based on their race:  https://www2.census.gov/library/publications/decennial/1920/volume-4/41084484v4ch01.pdf

Page 4, Table 2 of this document shows the main occupation categories of people in 1920 by percent:  https://www2.census.gov/library/publications/decennial/1920/volume-4/41084484v4ch02.pdf

Ethnicity: This chart shows the ethnicity numbers for 1920 (you'll have to look for the right year). You can calculate the percentage of each ethnicity by dividing the number of people by the total number.  https://en.wikipedia.org/wiki/Historical_racial_and_ethnic_demographics_of_the_United_States#Historical_data_for_all_races_and_for_Hispanic_origin_.281610.E2.80.932010.29

Location: Page 10 of this document shows which states had the most people living in them in 1920.  http://www2.census.gov/prod2/decennial/documents/41084484v1ch1.pdf#[0,{%22name%22:%22FitH%22},811]