Rx inserts have a fixed lens area in the front, but allow an insert containing the custom-made Rx lenses to slide in behind the front lenses. In this fashion, the possibility of water leaking into the mask is reduced greatly; though, depending on the depth, temperature and type of water that you will be diving in, fogging may be a bit more awkward with inserts.
Pre-made or specially-made prescription lenses - Most people who need visual modification have a comparable prescription for both eyes, and a dive mask with a pre-made Rx that is the same on either side generally gives them adequate vision for reading gauges and maneuvering around underwater. You can find a prescription mask from any category of pre-made prescription lenses, just like shopping for any other item of clothing or equipment already made to size, though pre-made prescription lenses do not have any astigmatism correction most people don't suffer when wearing a spherical (meaning "without astigmatism") correction in the water. Or you can have them made particularly for you. Obviously pre-made masks will cost less. Some older types, designed to accept Rx inserts that sit separated from the mask and the wearer's face, were custom made to allow prescription lenses to be slid inside, but these are notorious for leaking. Today the same model is being made, but they are usually very watertight and functional