How Do I approach the matter of detecting skin color, wrinkles, age estimation using ARkit / AR foundation in unity ? I haven't found any tutorials online that can help on this.
You don't, and can't.
Although ARKit/ARFoundation has face detection algorithms employed, it is only used for detecting landmarks and converting that into virtual space coordinates, of which you can scale/whatever your objects around.
ARKit/ARFoundation is a high-level API used for generating points; landmarks to map the virtual world space onto the real world.
You'll need to employ your own algorithm onto the image in order to do this.