They generally use whatever is available and cheapest. However, due to the constant power demand and the capital cost involved (you don't want your miners sitting idle), if that isn't grid power, its often either "reliable" renewables (e.g. hydro), "stranded" fossil energy (e.g. gas that would otherwise be flared off), or in the worst case, literally buying a coal power plant that was about to shut down just to power a Bitcoin mine.