I've been reading some public health papers and they account for demographic factors such as age, gender, and race in regression models (poisson/negative binomial) where the outcome is daily/monthly rates. Considering that the demographic factors are at the individual level and rates at city/state level, how is this done? If the demographic factors are considered at the city/state level, would they be percentages or number of people within those categories?

I'm sure they did not standardize/adjust for these characteristics prior to running the model because they clearly state that these factors were put into models as covariates and they modeled crude rates.

Thank you!