Every few years the National Fire Protection Agency (NFPA)--which is a non-governmental nonprofit--publishes an updated standard and then it's up to all the state legislatures to update the laws to require the new standard.

The problem is that the NFPA doesn't just post the NEC for free on the Internet. You have to pay for it (which is ridiculous) and it's copyrighted and as such, illegal to just give away. There was a lawsuit in Georgia that was supposed to determine whether or not that situation was OK (having laws that require expensive fees to view may be unconstitutional since that's like having secret laws). I don't know what the outcome was of that though.

The Supreme Court in 2020 ruled that OGCA is not copyrightable, this in 2022 indicates that it has been made available to the public for free.

There are four states who have not adopted the NEC on a statewide basis, leaving any such decision to the local authorities having jurisdiction: Arizona, Illinois, Mississippi, and Missouri.