Mr. Huh leads a project begun by the start-up accelerator Y Combinator to explore the creation of new cities. Hundreds applied to work on what looked like “the ultimate full-stack start-up.” Last October, Sidewalk Labs, an Alphabet company, announced it would team up with a government agency in Toronto to redevelop a stretch of the city “from the internet up.”

For others in tech — intrigued by word of a proposed smart city in Arizona, a big Bitcoin land grab in Nevada, a special economic zone in Honduras — fantasizing about newly built cities has become a side gig. They dream of utopias with driverless cars, radical property-ownership models, 3-D-printed houses and skyscrapers assembled in days.
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