Kind of a strange question, but let’s say that the apocalypse occurs, or at least solar flares that knock out the internet for months. What services would you need to self-host to be able to arbitrarily configure and build a NixOS based system?
Ideally, you’d point your system to self-hosted services, such as your own nixpkgs and binary cache, which get synced with public repos regularly. Is that all that is needed? Or would there be more to it? How would you ensure the cache has all needed packages? Presumably you wouldn’t be able to rebuild a lot of packages without internet. Or can dependencies be cached as well?
Search for “Carrington Event” in 1859. Were such a coronal mass ejection event to re-occur in modern days; it would be beyond catastrophic. The problem is not so much a loss of Internet as a loss of electrical power that could last decades. It would definitely fry any exposed electronics, especially semi-conductors. It would destroy power station transformers which are not easy to manufacture and would take many years to replace a large number of them. Providing you had power to run manufacturing.