Close but on the slightly wrong track.
The total amount of detail is determined by the number of pixels.
Whether the pixel is 'turned on' or not is, obviously the result of whether enough light reaches it.
In the shadow the amount of light that reaches the sensor is so little that the amount of energy from the received light is close to the 'noise' level common to electronics.
Thus there is a low (poor) signal to noise (s/n) ratio. This is why, if you artificially boost the shadows or an underexposed image, the shadows show a lot of noise.
ETTR is done to make certain that even the shadows get enough light from the lens that the s/n is good (high) and the shadows will have detail and no noise.
This means that the highlights have to be watched so they don't get blown out.
With a shot done ETTR the SOOC image will look too bright.
As long as the highlights aren't clipped, this shot can be adjusted to give a great image with nice shadow and highlight detail.
Learn to use the histogram in the LCD to see where the values lie.
