How ISO works
It doesn’t make the sensor more sensitive, but boosts the electrical signal from the sensor
Strong light creates more signal from the sensor, so less amplification or ‘gain’ is needed.
The ISO control acts like the gain control on an amplifier: the camera converts the light hitting the pixels on the imaging sensor into an electrical signal, which is then boosted according to the ISO.
Less light means less electrical current from the sensor. To avoid pictures being underexposed, the signal can be boosted.
The less the signal is amplified by the gain circuitry, the cleaner the picture will be, with less grain in the image.
The more the signal is amplified, the noisier the picture will be. At very high ISO settings, image detail will be obscured by coloured dots.