I read they either dump it into a local river while monitoring it or back to water treatment plant. Some datacenters have started reusing water but for some reason using it more than once is not appealing. Definitely the biggest problem is it’s not always a closed system and dumping it into a river potentially damages the river and makes the area lose their water. A percentage of the water is also lost via evaporation.

Source: https://dgtlinfra.com/data-center-water-usage/

Datacenters and fabs are different things.