Fetch registry content using API calls
podman and docker are cli clients that already send some headers to the registry and have their own logic; a lot of people integrate with the registries by directly sending API calls where they define accept headers.
Verify that the correct content( if available of course) based on incoming headers.
This section might be helpful to understand how registry interprets incoming accept_headers and what it decided to return. https://docs.docker.com/registry/spec/manifest-v2-2/#backward-compatibility