Imagine making ice manually. You fill a tray with water. You chill the tray until the water freezes. Then you push the frozen water out of the tray.

A fridge/freezer based icemaker works the same way, it’s just automated. A water valve activates to dispense a specific amount of water into a built-in tray. The tray is chilled by virtue of being in a freezer. Sensors in the tray wait until the water is below a certain temperature. Once it is, the tray is heated slightly to melt the ice a bit to make it easier for a mechanical device to eject the cubes from the tray. The cycle repeats if there is a lack of ice in the ice bin.