ISO Measures ???

Sorry but that is not correct.   The sensor does not become more sensitive to light when you select a higher ISO setting.

Imagine you have the shutter speed and aperture you want but the exposure meter is reading one stop under-exposed, then you can up the ISO setting by one stop and the exposure meter now reads correctly.

All the one stop increase in ISO setting has done at this point is move the exposure meter reading up one stop.  It does this because after capture the camera is going to amplify the signal captured by the under-exposed sensor in order to get the equivalent of a one stop increase in brightness.

However, the sensor is still one stop under-exposed and therefore the signal to noise ratio which will be obtained will be impaired compared to a correct exposure at base ISO. The ISO setting basically allows the user to take a picture with the camera sensor under-exposed if they cannot, or do not want to, decrease the shutter speed or open up the aperture.

Fred