It’s natural. I shave my head every morning, so it is smooth and shiny all day. For the most part, the shine on mine comes from all the oils that my scalp pumps out that would ordinarily end up in my hair. All I have to do is run my hand over my head and look at my fingers to see the oil.
Then again, a little Turtle Wax gives a nice sheen too.
I forget the brand name, but a few weeks ago while waiting for the slow pharmacy staff to find my pills, I stumbled across scalp stuff for head shavers - it was available in matte or gloss, to suit your mood that day.
On nearly every other part of the body, the skin get flexed, and stretch/fold lines form. On the scalp, it covers a big, immobile skull, so there’s very little motion to form irregularities. So, you get a smooth, shiny skin.
I can see this on my own dome (not shaven, my face just extends farther up than before). The forehead has a lot of use due to forehead muscle contractions over the years, but there is a line of demarcation where (I’ve always assume) the muscles stop and from there up (or back) it’s quite smooth where the little remaining hair doesn’t cover it.
Were this correct, most everyone would have shiny scalps, regardless of degree of hair loss. But looking at someone with thick hair, it’s obvious that the scalp is not shiny. Instead, I think the shininess comes from the atrophy and miniaturization of the hair follicle, which stretches the scalp tauter and makes it progressively shinier. My guess is that the scalps of people who are naturally bald are **all things being equal ** shinier than those who shave scalps that otherwise would have thick hair.