How do engine coolants work?
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Engines do produce a lot of heat. First as the compress the air which makes it hot, and then when the fuel is burned the temperature increases even further. If the engine gets too hot it will fail. The metal will warp or even melt. The lubricating oil will decompose or burn turning it into charcoal. Metal will expand so pieces will no longer fit together. Rubber hoses and such will melt. Even things like spark plugs may end up melting so they stop working.
So the engine can not get too hot and it needs to be cooled. Some smaller engines can be air cooled. Just letting enough air flow over it and fill the outside with cooling ribs for more surface area will cool them down enough. But this is not enough for larger engines like most of the ones you find in cars, and even medium sized bikes. Air is not good at transporting heat so we need much larger surface area. This is what the radiator is for. Tiny fins in the radiator makes for lots of surface area to carry the heat away. But to get the heat from the engine to the radiator we use a liquid. So the engine block have lots of coolant passages going through it and a coolant pump to pump the liquid from the engine block to the radiator and back. Some vehicles might have other things connected to the cooling loop. The liquid is basically water, but it is best practice to add things like glycol to it in order to prevent it from freezing if you park in winter and various other things to prevent it from rusting out the metal.
There are lots of little details to this system. The radiator cap is designed to let steam vent if the radiator can not keep up and the coolant boils. There is often an expansion tank so the coolant can expand when it is hot and shrink as it gets cold, although this tank is usually integrated in the radiator. The engine have freeze plugs to let the water out if it would freeze in winter. There is a thermostat to allow coolant to bypass the radiator if it is cold in order to heat up the engine to operating temperature faster and keep it at this optimal temperature. Some of the coolant is sent to the cabin heater to heat up the cabin if it gets cold. Certain cars have electrical heaters in them to allow for easier cold start, or they may have connectors to combine the cooling loops of a running car to a parked car to heat it up before starting it. There are lots of these details and concepts around the cooling system of a car.
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