I'm looking for a way to do that effect with Photoshop. It looks like noise but it's not.
If I zoom in:
I agree with user287001, that such an effect would normally be seen as a fault to be avoided.
However, it is possible to get such an effect deliberately if that's the look you want - for artistic purposes.
For the best results, start with a RAW file, and open in Photoshop. Then in Adobe Camera RAW, increase the clarity and contrast, so that you get fairly dark shadows. In the "fx" tab Add some grain to the image.
If you don't have a RAW, I suppose you could try Filter > Convert for Smart Filters, and then Filter > Camera RAW Filter - then apply the same edits.
Then open the image, go into LAB mode (Image > Mode > Lab Color). Add a Levels adjustment layer, and in the Levels dialog select the "Lightness" channel, then move the left slider towards the middle of the levels histogram, and move the middle slider back to the middle.
Here's a screenshot showing the levels adjustment and the effect achieved.
It's actually a fault. Heavy luminosity compression is needed to have some details other than black in the shadow. Local contrast is added to lift up the details. Unfortunately there is only 2 luminosity steps information left in the shadow. The noise makes fine dithering that creates an illusion of greater bit depth.
One gets easily to this by trying the RAW image treatment to a photo that has only the ordinary 8 bits/color and deep, nearly black shadows that need to be revealed.
Here is an example of the shadow lifting on the right side
Some raw developing programs create this without asking if not carefully adjusted. Their "one take HDR" makes it. Adobe Camera Raw is extremely carefully designed to avoid this.
Filter>Pixilate should be in there
We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.