For anyone who winds up here,
I tried to use Developer Authenticated Identities in Cognito
I was unable to authenticate AppSync calls from this method as I found the documentation lacking and I had too much uncertainty over how to proceed.
I also looked into using Shopify as an OIDC provider directly with Cognito. This is not supported by Cognito out of the box, as each Shop has their own OAuth URLs. Cognito expects a consistent OAuth URL per provider, so you will have to create a custom Cognito Auth flow. I found some documentation on that here https://www.richdevelops.dev/implementing-shopify-oauth2-with-cogntio-user-pool-amplify-and-lambda
I decided not to pursue any of this at the moment as it is time consuming, and I am stalling out on this.
I instead opted for traditional username/password logins with the Amplify SDK, and that’s working well enough. It’s working well enough.
Good luck!