Generally these informations are not published publicly. If you/your customer is keen to know more about it, suggest kindly reach out to the AWS Compliance team AWS Compliance
Am sure, there would be certain information shared directly using this NDA route
IF you think of an AZ as a Datacenter, or multiple DCs within 10-20 miles of each, I would consider a region separation of about 100 miles. You can stretch a VMC cluster across multiple AZs in the same region. If you want to protect yourself against a region failure, you'll need to setup another cluster in another region and look at SRM.