It depends on the state and jurisdiction. It is correct, that some indeed are approved on the state level, but also on the local level. When new cities are created, one of their first orders of business is the adopt the rules of the community, it city code, zoning codes, etc. This should be done as the very first act once they establish the government.
Now, what typically happens is that they adopt the regulations of the previous jurisdiction. So a new city, will adopt the codes of the County as their own. It usually takes a few years until they are able to take the time and adopt their own, but there should be no break in regulation. They will also likely keep administration of those codes with the previous jurisdiction on a contract agreement, until they can get sme departments up and running.
What happens if they don't do that? Your fire hazard insurance rates will sky rocket. You can expect to pay 5-10X your current hazard premiums on your mortgage, effectively almost immediately. How many people can handle that? Much less carry those numbers for more than a few months? The lack of an adopted fire code / building codes, unless you are already in a community that doesn't already have them, is a major impact to the residents. There are many jurisdictions in rural Texas and Alaska, where there are no zoning regulations or building codes, and they do pay a premium for their insurance.
Once incorporated, they also need to start working on a comprehensive or general plan for the community. It is based on those goals and visions where the zoning codes would then originate from.
Incorporation of new communities is a major process. It takes time to accomplish. San Tan Valley here in Arizona has been trying to incorporate for almost 20 years now, unsuccessfully, mainly because of opposition from the surrounding jurisdictions.
