Azure Regions, AZ and Region Pairs

Question 1

Fault domains

Answer 1

3 by default
Separate server racks which have separate power etc.
Your VM is deployed to say all 3 fault domains and then if a fault domain goes down your still good on the other 2.

Question 2

Update Domains

Answer 2

5 by default
when your vm might need updating, this concept means that some copies can be updated so that others stay up

Question 3

Availability Set

Answer 3

Availability Set distributes highly available workloads across multiple Fault Domains, thereby eliminating any single point of failure.