Aluminium foil is made a lot like pasta: you put an ingot in a mill, then rolls press it from both sides over and over so the ingot becomes thinner and lengthier. And at the final step the foil is so thin that you need to mill two layers of foil at the same time, otherwise it'll break. The side of the foil touching another layer of foil becomes dull. And the side touching the roll becomes shiny.

Note that for all practical purposes both sides behave the same.