/3/ - 3DCG

>>740032
The Red channel in the maps are just masks to differentiate between things like clothing pieces and the skin. It's not very important unless you're planning to, say, generate a repeating skin bump texture on the head mesh and need to prevent bumps from covering the lips, which you would go around doing by plugging the (_B)'s red channel into your Factor input.

See screenshot for a very basic setup of the files. I don't use the ambient occlusion (_O) map because anyway you're using Eevee's own Ambient Occlusion. As for the blue channel of _M, it's a Cavity map which you can use by mixing them with other maps to darken/lighten the crevices. It's entirely optional, and it's up to you to decide what you want to do with it.
>>740048
Cut the Alpha, plug in the Color output to the Alpha. Then manually set the Base Color to black.
Also set your texture's Color Space to "Non-color", since black/white and normal maps tend to require those (yours is no doubt an AO map).