Different devices to refill the evaporated aquarium water. Such a system is also called an ATO, auto top-off or osmolator.
The price difference between the various systems is due, among other things, to the protections that were used. One system has no protection if a sensor does not work. Another system uses an optical sensor and a float.
Always use osmosis water to top up the evaporated water as the salt does not evaporate.