Accessing existing S3 buckets from Amplify app

[timjadams]

There is a lot of documentation showing (different) ways of configuring Amplify gen2 to access an S3 bucket, but I have yet to see how the implementation is completed in code. I just want a simple backend function that will list a bucket contents (then read and possibly delete a file) in S3. I can not for the life of me get this working using any of the examples I have found, and I can find nothing on the wider net. It cannot possibly be that an amplify app can only use S3/Cognito/etc resources that are configured by the app generator, but that is all I find anywhere.

Cognito cannot be used as a SSO if a new user pool has to be created each time.
A database and storage cannot be shared if I cannot access them from the apps.

How is this done?