It sound like they're using land that Google already owns for this: "First, over the next 10 years, we’ll repurpose at least $750 million of Google’s land, most of which is currently zoned for office or commercial space, as residential housing. This will enable us to support the development of at least 15,000 new homes at all income levels in the Bay Area, including housing options for middle and low-income families. (By way of comparison, 3,000 total homes were built in the South Bay in 2018)."