At the time of writing in April 2018, the most common number of AZs in a region is 3. The next most common number is 2.
The following from the S3 FAQ seems to be inconsistent with that:
Q: What is an AWS Availability Zone (AZ)?
An AWS Availability Zone is an isolated location within an AWS Region. Within each AWS Region, S3 operates in a minimum of three AZs, each separated by miles to protect against local events like fires, floods, etc.
My assumption is that there is a hidden geographically isolated facility where required.
Sometimes the course notes do in fact use the word “facility” eg:
By default, data in Amazon S3 is stored redundantly across multiple facilities and multiple devices in each facility.