Thanks for your inquiry.
BME280 pressure sensor working principle is at https://www.youtube.com/watch?v=zXIeqeT_FC8. It is pizzo resistive Winston bridge.
BME280 humidity sensor is based on capacitance change MEMS technology and the temperature sensor is from the diode on the MEMS structure.
BME280 = BMP280 + humidity sensor. BMP280 working principle is decribed a little on page 3 of BMP280 datasheet at https://www.bosch-sensortec.com/media/boschsensortec/downloads/datasheets/bst-bmp280-ds001.pdf. Due to IP concern we cannot disclose more about BME280 humidity part working principle.