They say it’s harder to get into than Harvard: Y Combinator, YC for short, is “startup school” for tech founders. It takes applications twice a year. Being among the 230 startups accepted out of 24,000 means getting a half-million-dollar investment and access to mentors who’ve already made it. Airbnb, Reddit and DoorDash are on the alumni list. For most of its 18-year history, Y Combinator has been based in Mountain View, California, the heart of Silicon Valley. Recently, though, its center of gravity has moved about 40 miles north to San Francisco. YC opened a new office in June and now considers the city its headquarters. Garry Tan took over last year in a role once held by OpenAI CEO Sam Altman. Tan wants founders to be nearby, at least during the first three months they’re in the program. He told Marketplace’s Lily Jamali why during a walk through the city.