to finally answer your question I use Clipstudio Paint. And the patterns are small dither textures ( clipstudio link for example: https://assets.clip-studio.com/en-us/detail?id=1719532 ) I also apply more dither through an option on Clipstudio when exporting my GIFs. but I'm sure if you google the software you use + “dither textures” you can find some to download
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