I know nothing about koi ponds. Sorry. I know a lot about water quality. And life support. But keep googling. I’m sure you’ll find some good information. Unless any of my awesome followers know some things. They’re really nice.
You need roughly 1000 gallons of water per koi, they get very large. Keep in mind they’re pretty much just decorative carp. If the pond is well filtered and planted, it’s 1000 gallons for the first koi, then 500 gallons per additional koi. The pond should also be at least 3 feet deep in spots to provide protection from temperature shifts. They aren’t really any different than other fish, aside from that. Make them a large pond, equip a strong filter, put in some aquatic plants, feed them a decent quality food (less in the winter), don’t let the pond ice over completely for too long, keep herons/cats/raccoons/etc away, and that’s about all.