That's a region. It's not only many buildings, it's many zones, each of which are many datacenters. A region is just a virtual partition for their services. A zone is a fault domain for their services, and a single zone is met by many datacenters, each of which can have many buildings. Or at the least, I know of at least one datacenter which has multiple buildings, that is within one zone that has multiple datacenters, that is within one region that has multiple zones.