From what I can see, it was Canyon, ID, which has a population of 230K and is located west of Boise. Is there a more populous one anywhere in the country?
Btw, the most populous county where Biden won every precinct was New York, NY (Manhattan).
I believe Trump won every precinct in Jefferson County, MO and St. Charles, MO in 2016.
For 2020, I think he still won all Jeffco precincts but maybe lost a few in St. Charles. I will have to check.
Lost some in STC but JeffCo held on. Closest one I could find was 56-42
