One aspect I've learned about is what I wrote in a different comment: Cells in different locations produce different chemical markers. These markers spread out

One aspect I've learned about is what I wrote in a different comment: Cells in different locations produce different chemical markers. These markers spread out through the tissue and create a sort of gradient. By observing the gradient, a cell can know where it is (or at least what it's supposed to do). At least that's the explanation as I've understood it.