[jmiserez]

No magic and the same photons, but you can have the hardware sensor read them out differently. Specifically using varying amounts of analog gain / amplification before doing analog-to-digital conversion, minimizing noise. This varies based on camera design.

See the "ISO-Invariance and Downstream Electronic Noise" part here for a better explanation: https://www.lonelyspeck.com/how-to-find-the-best-iso-for-ast...

The article mentions the Sony A7S as an example, with the sensor showing marked improvements in SNR when reaching ISO 100, 200, 1600 and 3200, while behaving ISO-invariant wrt. noise in between those values.