At the 1 minute mark of the "Database HA options" video, two (2) stand-by MariaDB instances are depicted. However, according to the RDS FAQ "Amazon RDS provisions one dedicated standby per DB instance primary". For more details, please see "How do I control/configure Multi-AZ synchronous replication?" question of the "Multi-AZ deployments" section at https://aws.amazon.com/rds/faqs/#Multi-AZ_Deployments
Please can you clarify?
Yes, to be exact, you are provided only 1 standby in a different AZ. The diagram was intended to show the concept of redundancy in those other AZ’s as we don’t get to control which other AZ our standby lands. Behind the scenes, if the AZ that contains our standby has a problem, the standby will be moved to another AZ.
There is a great blog post on the behind-the-scenes workings of multi-AZ RDS here: https://aws.amazon.com/blogs/database/amazon-rds-under-the-hood-multi-az/