The softness you feel when you run your hand over a duvet cover starts not in the shop but in the field. The length of the cotton fibre is the first thing that decides both how a fabric feels against the skin and how many years it will last.
Think of it this way: when yarn is spun from short fibres, more fibre ends are left loose on its surface. Those ends feel slightly rough to the touch, break off in the wash and turn into the familiar fluff. Long fibres wrap around each other more tightly. The yarn becomes both finer and stronger.
That is why Egypt's Giza and America's Supima are famous. Aegean cotton from Turkey, grown not far from Denizli, holds its own in this league in a good season, and it deserves the credit. Then there is the word "combed", which you often see on labels. Combing removes the short fibres and leaves the long, even ones. "Carded" yarn goes through a lighter version of this process and is more economical. So "100% cotton" on a label does not tell you much on its own. Which cotton, and how it was spun: that is the real question.