I found this tutorial had some very good information on incorporating Cognito into an app: https://aws.amazon.com/blogs/aws/build-your-first-serverless...

It lays out the architecture fairly well and how Cognito ties into it. Still very fiddly, and doesn't give any indication on how to use their hosted forms (which are a bit of a mystery to me as well).

Since it's still on point with the OP's question, I'll ask: What has your experience been with auth0? It's another service I have considered.