That’s true, but my understanding is the main contributor is skin effect, since AC travels only on the surface of the wire, while DC uses the whole area, result

That’s true, but my understanding is the main contributor is skin effect, since AC travels only on the surface of the wire, while DC uses the whole area, resulting in lower resistance loss (https://en.wikipedia.org/wiki/Skin_effect https://en.wikipedia.org/wiki/Skin_effect)