That's interesting to know. But since it's space, how do they then cool down the hot fluid?

You cool the fluid by flowing it through the radiator. The radiator emits heat radiation into space and cools down the fluid. As long as the fluid is hotter than the equilibrium temperature of the radiator (determined by radiator, space and sun radiation), it will emit more energy than it receives and cool down the fluid.

Blackbody radiation, see (https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law)