Amazon Certificate manager Distribute across regions

Adding to the answer from Junhwah An:

The certificates generated by ACM aren't retrieved in real-time from the service by the components that are using them (CloudFront, ALB, etc.). Those certificates are loaded when they are generated and then used until they are replaced or expired (if you cancel the automatic renewal).

So a network outage won't affect the certificates being used unless that outage is long enough to block the renewal process.

I'd also add that we don't use the internet for connectivity between regions - we have a private network for that: https://aws.amazon.com/about-aws/global-infrastructure/global_network/

AWS Certificate Manager is a regional service of AWS*.
And the physical and logical separation between AWS Regions avoids correlated failures between them**.
But if you use an ACM certificate for Amazon CloudFront, your certificate has a dependency on the Northern Virginia (us-east-1) region.

* https://aws.amazon.com/ko/about-aws/global-infrastructure/regional-product-services/
** https://docs.aws.amazon.com/whitepapers/latest/aws-fault-isolation-boundaries/aws-service-types.html#regional-services