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?
https://en.wikipedia.org/wiki/Image_noise
Does a better example of explaining it in depth than I can.
That’s a great resource, here is another that has more visuals on sensors and how they work. https://graphics.stanford.edu/courses/cs448a-10/sensors-noise-14jan10-opt.pdf
Edit: Page 36 is where signal to noise ratio is directly addressed.