I will try answer both your questions on the fridge here and I think to answer this, we have to understand how a fridge works.
The fridge works by compressing and expanding a gas as it flows through a continuous pipe. By compressing gases, we make them hotter; by letting them expand, we make them cooler. The compression of this gas is done by an electric pumping machine at the back of the fridge, which produces the humming sound the fridge makes.
Items in the fridge do not cool by lowering their original temperatures; instead, the refrigerant ie the gas draws heat away, leaving the surrounding area much colder. Refrigerators and air conditioners both work on the principle of cooling through evaporation.
The gas in the fridge when compressed becomes a liquid. It never runs out-unless there is leakage, because it moves through a continuous tube even through the compression-evaporation cycles. In the past, gases such as ammonia, methyl chloride and sulphur dioxide were used which are toxic if they leak. Nowadays, the gases used are mostly not harmful and include halogenated compounds of methane.