It not warm as in ”warmer than the typical living space”, but it is warmer than zero Kelvin, so heat can be extracted from it.

Doing that takes energy, that’s why it is called a heat pump. That moves heat from the water to an already warmer place, against a heat gradient, just as a water pump moves water against a gravity gradient.

If the water were warmer than your typical living space, they wouldn’t need a heat pump; a water pump to pump the water closer to where heat is needed would be sufficient.

The river is not warm or warmer than the air. Heat pumps are amazing at extracting thermal energy. I think water is very dense compared to air, thus making the processes more efficient in such a large scale.

It has a decent bunch of thermal mass, so it takes quite a long time for it to reach air temperature during a cold snap or heat wave. This makes it a decent heat source during the winter and cold source during the summer - especially for short-term peaks.

You could get an even better result using the earth itself, but that is way harder to scale.

It isn't. It's just warmer than air in winter