I know that BME280 measures pressure using the piezoresistive effect, I suppose it uses the thermoresistive effect to measure the temperature. How about humidity? Any advice would be appreciated, even starting points for a little Google-fu.
Typically low cost RH sensors use capacitance measurement with a hygroscopic polymer layer.
See, for example, O’Dowd, J., Callanan, A., & Banarie, G. (n.d.). Capacitive Sensor Interfacing Using Sigma-Delta Techniques. IEEE Sensors, 2005. doi:10.1109/icsens.2005.1597858