I know, I know, it can be caused by various factors, like high ISO, poor lighting conditions, long exposure time, heat. (not that I know why the last two cause noise)
But where does noise REALLY come from? What is noise? What does it have to do with photons and photosites?
Noise can come from the gain stage - where electricity is used to control the “ISO” of the sensor. Or it can come from the encoding stage - the codec turning raw bytes into a digital file.