As far as I can tell, all the talk of noise and such here was concerning RAW, not JPEG. Lossy compression introduces many artifacts that have nothing to do with the noise being talked about here.
The ISO setting in the camera is different from raising the image brightness digitally in post. ISO is amplification of the electrical signal
before it is converted to digital. What that means is that the noise in the image is also amplified, but any noise that is added through the electronics in the remaining steps leading up to a digital image is not amplified. If you do all your brightness increase in software, even the noise introduced by the electronics involved in the ADC get's 'amplified' (digitially). Especially with older sensors (anything older than the M6 II, 90D, 1DX III, R6 or R5), this will lead to an image looking siginficantly worse if shot at a low ISO and raised in post rather than just shot with a high ISO in the first place.
Feel free to run the experiment yourself, if image quality at high ISO is a topic of interest to you. I have also done exactly that a little while back and tried to share the results and my understanding of the theory behind it in this post here:
Introduction An image says more than a thousand words! Alright, so show, don't tell! ;) What's up with all this talk about equivalency that seemingly derails every thread around here? I'll be showing you actual images that illustrate what I am basing my post on, when I participate in these...
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