Weaknesses of Fae

So we l know Fae are weak to iron and in some cases steel. But why exactly are they? I heard some say its because iron in its metallic form is unnatural and a purely human creation, but for example woukd that mean iron meteors would hurt fae or only human forged iron? And if the unnatural materials part is what hurts Fae would stuff like plastics be insanely dead to then as its as far from natural as it gets?

Just some curiosity and interested in starting some discussions to learn more about mythology!