A single AZ may have neighboring datacenters, but they are very strict on having datacenters for different AZs be at least 100km apart and on different flood plains and power grids.
Each Availability Zone can be multiple data centers.At full scale, it can contain hundreds of thousands of servers. They are fully isolated partitions of the AWS global infrastructure. With its own powerful infrastructure, an Availability Zone is physically separated from any other zones. There is a distance of several kilometers, although all are within 100 km (60 miles of each other).