So, you’re wondering about humidity sensors? Well, let’s start by going to Wikipedia to get some background information on relative humidity. Ah, interesting, the article mentions that a device for measuring humidity is called a hygrometer. And indeed, the mechanisms of a hygrometer are very similar to some of your suspected ideas. One method may involve measuring the rate of static electricity dissipation, other methods involve measuring capacitance or resistance.
20181129/https://en.wikipedia.org/wiki/Relative_humidity
20181129/https://en.wikipedia.org/wiki/Hygrometer