The data is obtained from serval sources. The spatial data is mapped onto Zip Code Tabulated areas (ZCTAs) level. It was chosen to plot the data on a ZCTA level as alot of data is available at this level, the area covers most of the population in California, and has a much finer resolution than County level.
ZIP Code Tabulation Areas are generalized areal representations of United States Postal Service (USPS) ZIP Code service areas. The USPS ZIP Codes identify the individual post office or metropolitan area delivery station associated with mailing addresses. USPS ZIP Codes are not areal features but a collection of mail delivery routes. ZCTA on the other hand is an areal feature which origins from USPS ZIP Codes but is only updated every 10th year.
ZCTA only exists for areas which are densely populated, which explains why data cannot be plotted for deserts, mountains, etc. of California. For further information regarding ZCTA please visit: https://www.census.gov/programs-surveys/geography/guidance/geo-areas/zctas.html
The data for the Election results was originally estimated on a geo Block Cencus level, where we have chosen to remap this to ZCTA level in order to match the other datasets.