How does the TTN server integrate with AWS EBS?

The doc (https://www.thethingsnetwork.org/docs/applications/aws/thing-registry.html) provides a high-level description of what the integration does, but it doesn’t specify how the integration is done.
Are TTN servers integrating with AWS EBS via REST APIs or some other communication protocol?
How is the communication secured?
How do we ensure that only TTN servers can access the EBS end-point?

Why isn’t the EBS application source code in github? (it’s only available compiled on S3
https://s3.amazonaws.com/thethingsnetwork/builds/integration-aws/dist/integration-aws-latest.zip).

That application works well as a proof of concept but would require more detailed specifications for a production load.
For example, has anyone fronted the EBS application with API Gateway?