Distilling/deionizing water is indeed quite trivial.
But I personally know a beer brewer that was "remineralizing" water to a certain specification, and at some point they stopped because it was far too expensive. So I'm not so sure about the claim that people can make water with whatever properties they want, at least not in a way that is broadly economically viable.
But I personally know a beer brewer that was "remineralizing" water to a certain specification, and at some point they stopped because it was far too expensive. So I'm not so sure about the claim that people can make water with whatever properties they want, at least not in a way that is broadly economically viable.