[toomuchtodo]

AZs in AWS are essentially distinct "datacenters" from a logical perspective. There is no shared infrastructure between them; if AZ B drops off, as long as you have your data and instances replicated and serving in another AZ, you will see no downtime.

[mentat]

That is "in theory". In fact there have been region wide failures at the AWS services level, though none recently that I know of.