“And where did all that water go?”
Evaporative cooling is the norm in low humidity areas for not only datacenters but also most commercial HVAC chillers. Water is 8000+ BTU per gallon just in vaporization energy add in the temp change from ground water temp of 55F and it’s easily 9000 BTU per gallon. A ton of HVAC is by definition 12,000BTU per hour.
So yes it is evaporated into the air not returned to a river or sewer system. It will eventually rain out somewhere having been returned to the overall water cycle it is however lost to the local basin and users.
Closed cycle cooling is possible using dry towers it uses a lot more energy and also is much more expensive so wet towers are the first choice. You don’t have to use potable water , grey water or purple reclaimed water works just as well. San Antonio has the nations largest purple water network, Vegas is second both supply reclaimed water to cooling uses,golf courses,parks and industrial uses.
Aalo is addressing both the cooling and power uses for d datacenters with their nuclear reactors providing electricity on site and the waste heat is used to drive adsorption chillers with dry cooling towers for those and the reactors turbines too. They have their first reactor in prototype wanting on the NRC to let them fuel it. They have their first factory to mass produce them set up too.
On-site nukes, dry cooling and hear driven chillers is obviously the way to go.