Not all pools have them. An in ground pool would have one somewhere near the top of the tile line. It is there so that the rain water will not go over the coping onto the deck. Inside the skimmer is a place to look on both above and in ground pools. k
I've seen 1000 pools or more, but never an overflow. I'm guessing you are worried about rain over filling the pool. If the water level rises too much you connect a discharge hose to the filter or sometimes there will be a hose bib installed between the pump and filter. Attach a garden hose, turn on the pump.