When you're shooting JPEGs, ETTR is severely limited because of the way cameras' JPEG engines transform RAW data into JPEG image data. The ISO setting influences final image brightness but it cannot possibly change exposure, nor can it adjust the sensor's inherently non-adjustable light-measuring sensitivity, to accomplish the brightening. Notice that the ISO setting is not an exposure setting. By now it should be obvious that converting RAW data to image-file data cannot possibly change exposure. Transformations that happen in #6 are accomplished either by the camera's JPEG engine or by RAW processing software. These transformations happen after the shutter has closed and the light has been measured. Raising the ISO setting above base ISO affects #5 (when signal is amplified) and/or #6 (when a different tone curve is applied). Anything that happens after the shutter is closed and the light has been measured is manipulation of the measured values. Flash, by increasing luminance, also increases exposure. Exposure settings on your camera change aperture and shutter speed.
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