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?
Typically noise is deviation from the expected value. Expected value can be measured by taking multiple samples and the more samples you get, the more precise the expected value will be. In practise expected value is the average of the samples. Thus noise is deviation from that. Signal to noise ratio (SNR) tells you how much noise there is.
Light itself is noisy - light particles, photons, hit the image plane randomly, following poisson distribution. Thus the image that the lens draws is in itself noisy. The more light particles you collect, the larger the SNR will be.
Almost all the noise in the raw-file data comes from randomness of light.
Image sensor adds a tiny little amount to this and it’s irrelevant unless very little light is present.