Excuse me, how to make the background graininess in the picture with ps?

Select the area where you want to add points, and right-click in the selection-by copying the layer (or you can copy the layer by ctrl+c first and then ctrl+v).

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Change the foreground color to black and open the filter library.

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Click on the sketch-click on the halftone pattern again, adjust the parameters and confirm.

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Then adjust the parameters on the layer, and it's done.

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I gave an example:

Or, you can create more effects by changing all the colors in the foreground and background: