Thanks for the explanation of the detectors. I thought that one photon probably would not create enough potential at the input to the ADC to be measurable above all the noise in the detector - such as the thermal, shot, and other junk noises, or the noise in the connection to the ADC, and the ADC itself. This is why I thought there might be pixel-by-pixel amplification.
I think only one sensor maker (Sony) makes a sensor with on-die ADC. I could be wrong about that. I’ve seen some pretty nice looking astro-photography shots done with the Pentax K series (which is CMOS Bayer) - but professional astronomy is perhaps an order of magnitude more demanding?
Here’s an info link to the columnar ADC that I think Sony might be using. Interestingly, the Pentax K series that’s known for hobby-astronomy photo use employs a Sony sensor - probably with the columnar ADC, which may explain the high ISO capability of those cameras: