Note 4 ISO the mechanics of or where does the 'noise' come from.

Noise in digital photography as grain in film, but this noise comes from a different source than digital, needs to be considered.
Noise in film comes from the chemical process of silver-halide crystals in the films emulsion. Whereas in the 'film' age it was a 'tool' in the production of an image, I think digitally it is more of a 'nuisance'. As most photographers now want a smooth, clean image; noiseless.
Noise in digital comes from two sources:
Either or both are present to a lesser or more degree.
'Shot' noise is created by the basic variations in light.
'Electronic' noise, as its name suggests derives from your digital cameras internal processing.
Now we are not though saying that noise is bad and needs to be eliminated completely. No sometimes it can enhance the image just as grain did/does the print.
Most times though it is obtrusive.
Sometimes it is a trade-off in the exposure process.
SHOT noise is down to the makeup of light. Light is a packet of particles , photons. These photons reflect off the subject with a delay which our brain filters them out every day. Quite simply light falls on a subject da da da da not dadadada
This is the nature of light, technically 'the quantum nature of light'.

ELECTRONIC noise comes from in camera processing the light that falls on the sensor.
At this stage we need to know that changing the ISO does not influence the sensor. It influences how the camera processes the light. To process the light the set up is:
Sensor>Amplifier>Analogue to Digital Converter>Processor>Image File
The Amplifier reads from the Sensor creating Analogue Data which is then processed through the A to DC sending digital data to the Processor giving an Image File (RAW/JPEG) It is this, electrical, process which creates the Electronic Noise.

To continue, viewing light with the eye is not a problem, as the brain sorts it all out; normally until the sun goes down but then we are in the realm of artificial light, or very weak ambient light.
When your camera, more precise your cameras sensor has light falling on it the light falls in various intensities requiring different exposure settings.
Called the 'dynamic range' Highlights, Middle Tones and Shadow.
Noise is most noticeable in the shadows. If you exposure for the shadows you 'burn' out the highlights which affect the mid tones.
To minimise shot noise try to make your exposure as accurately as possible. Digital cameras are catching up and some higher end cameras shoot in HDR mode but not ideal for all subjects.
In film photography the saying for exposure was; exposure for the highlights let the shadows take care of themselves.
But back then film had/has a higher dynamic range, to date it still has, than digital has today.

To minimise 'electronic noise' (known as 'read noise' because the sensor 'reads it') this 'read noise' is now dealt very well in modern digital cameras but the work goes on.
And as camera technology improves along with improved software the photographer is able to handle 'read noise'.
In the future it seems because of the nature of light we would assume that technology would eradicate 'read noise' as the manufacturers have come a long way so far. 2017
One major improvement, outside the camera is HDR software. Which is also available in camera. In camera HDR will only produce a JPEG file.
But technology advances all the time.
To minimise 'read noise' easily is to look at the Dynamic Range of your subject, expose for each area of range which will give you a number of images taken of the same scene but with the exposure taking into account the varying intensities of light. (Dynamic Range) These images can be put together into one image through an HDR stand along software.

As this is being written manufacturers are developing ISOLESS Cameras. Will this mean we no longer need to know about 'where' noise comes from? As a photographer light is what you 'paint' with and having a better understanding of light might help you to turn that image into an Iconic image?
Manufacturers have given the photographer so many options in the matter of ISO/Noise.
In camera technology, noise reduction software, HDR software, image editing packages.

The information here is taken from The Amateur Photographer 04.02.2017 Full article pages 12 - 37
Para-phrasing it as I, as a photographer, understands the subject.
Please if you can, read the full article. A really super article.