While looking into the BME680 for a fun little indoor air quality monitoring project, I found this neat article showing what a humidity sensor looks like under a scanning electron microscope (SEM). The linked article itself was found from this hackaday article comparing the BME280 (like the BME680 but no MOX sensor) with other popular humidity sensors, such as the DHT22:
https://hackaday.com/2017/01/03/humidity-sensor-shootout/