How to quickly connect Amplify and local resolvers for out-of-band changes?

Struggling to understand how to quickly and easily create real-time updates from backend services (step functions, eventbridge, etc). There are numerous good posts on creating out-of-band appsync updates using sam or cdk.

For example:

https://aws.amazon.com/blogs/mobile/appsync-eventbridge/

https://aws.amazon.com/premiumsupport/knowledge-center/appsync-notify-subscribers-real-time/

https://serverlessland.com/patterns/appsync-lambda-graphql

As for amplify, is it a matter of creating a custom resource and using cdk to override? Is there an easier way to integrate?