How do you create squared big pixel texture like this in photoshop?

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Hard to be certain what you're looking for; one way to get something like this would be to use the Texture > Patchwork filter, with relief set to 0, square size to suit your needs. |
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I would say a grid is constructed using shape tools, then various fills, textures and filters are used to create the look thats desired, then it is probably fiddled with so that it is perfectly tile-able - using the offset filter |
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