It's usually not done on the sensor - it's usually done by the ADC that performs sensor readout.
Some cameras use a technology like Aptina-DRPix to dynamically change the capacitance of the sensor FETs, but (as far as I'm aware) this only exists in a simple binary form right now. E.g. one of my cameras reduces sensor gate capacitance when the ISO exceeds 800, but otherwise any ISO changes only affect off-sensor hardware.
You're still multiplying what is the photo-count or shotnoise
Didn't know there is technology built on top of that... Does changing the capacitance increase the sensitivity somehow? I guess then the question is.. why isn't that always enabled. There must be some downside to it
The pixels output voltage is the charge (number of photoelectrons) divided by the integrating capacitance. Reducing the capacitance increases sensitivity at the expense of full-well capacity - because the saturation voltage is reached earlier. That’s the point at which highlights blow out.
The advantage of increased sensitivity at the integration node is that all downstream noise is applied to a larger signal, so SNR is improved.