I don't even know what it would it even look like to "build a city" from scratch in the US. who does the building and puts together the central plan?

does the government build a bunch of public housing and a publicly owned commercial district? i guess they kind of have experience doing this with military bases, but at some point you need to encourage a bunch of private development and ownership, right?

or would the government just incentivize private developers to start building in the middle of nowhere and hope that a city arises as an emergent phenomenon? that approach seems like it would be rife with abuse and waste.

seems like this would be a lot easier to do with an authoritarian regime that could just decree "we're building a city here. the following industries will move their headquarters"

It's not particularly difficult to start a new city.

The government simply asks large companies to open offices/factories in the new city in exchange for tax breaks/subsidies. Or give funding to a university to open a satellite campus. All you need is a promise for like 20k people to initially move. Then the government builds roads and utility networks. Private developers will also build housing if given the right financial incentive.

The 20k people will automatically lead to the same number moving in due to cheap housing, or for creating every day businesses, hospitals, schools etc. Within a couple of years you can setup up a feedback loop where the population is growing at 5-10% every year. There is no need to force anyone to do anything. Financial incentives are enough.

Starting a city is easy, growing it into a real city is the hard part. If you look at the fastest growing cities of the last decades, they had economic freedom or booming industries, nothing that requires authoritarianism.

The western approach would almost certainly be a public-private partnership; we do that with all meaningful infra projects, where multiple industry consortia put together proposals and then one is selected to move forward. For example, for the ION Light Rail in Waterloo Region (~$1B), the winning consortium was composed of engineering and construction firms/consultants, a operations company that would run the system, plus a financier: https://en.wikipedia.org/wiki/GrandLinq

That said, for a project the scale of building a city, I can imagine it might actually be faster and more efficient for the government to just plan and build everything itself and then sell it off to private entities later.

Honestly, if you build transit, developers will build.

I wouldn't call it "building a city", but if you look at Northern Virginia today, you'll find that vertical districts are popping up along the Silver Line metro that now extends past Dulles airport.

At the end of the metro, there is literally a "town center" residential area on one side with buildings around 5 stories tall. On the other side of the tracks is literally fields, but the roads have been laid out like Sim City with empty plots and developers are now beginning to construct buildings starting from the outside perimeter first, working their way toward the metro station.

Throughout the DC suburbs, you will find densely populated areas with relatively tall vertical buildings (15-20 stories) that simply were not there 20 years ago. Reston is a good example. I've watched 4-6 buildings (over 10 stories) get built in Reston alone. They mostly started when the the metro line was finished.