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?
The source is quantum in nature. Every now and then an electron will break through and light up the pixel. Not much can be done about it happening, but software can “fix” things sometimes.