In the HR202 have graph resistance over humidity.
In the circuit humidity sensor module it shows have a pull-up 10K. So, it be a voltage divider.
Microcontroller just read analog voltage. Then, from analog voltage convert to resistance using voltage divider formula.
From the resistance convert to humidity. From the tabulated table, you can get the formula by using Curve Fitting.
Ex curve fitting
HSM-20G and
Maple Curve Fitting.