>>297565373
Most shaders like xstoon interpret the AO texture as how shaded that part of the model should be, along with the usual lambertian NdotL. half lambert is a common tweak to make the shadows less dark in general https://developer.valvesoftware.com/wiki/Half_Lambert

The SSS implementation in xstoon and others is a pretty cheap approximation, it's not raytracing or anything. It uses a "thickness map", which you can generate by flipping the normals of your model and baking AO https://www.alanzucconi.com/2017/08/30/fast-subsurface-scattering-2/