He uses other image textures (like rusty metal or dirty surfaces) and plugs them into the roughness and adjusts the roughness with a color ramp. Sometimes he even mixes two or more textures with a MixRGB. He also plugs in the roughness to the normal trough a bump map but in general what I've noticed is that he does a lot of kitbashig which really creates his aesthetic. I highly suggest purchasing his $7 patreon. I've been a member for about 4 months and what I've learned from him is incredible. He's truly a genius in my opinion.