About the grant type for the USER_SRP_AUTH in AWS Cognito

The USER_SRP_AUTH flow is used in the InitiateAuth API call (as described here) - https://docs.aws.amazon.com/cognito/latest/developerguide/amazon-cognito-user-pools-authentication-flow.html

This is an alternative integration method to login directly to the user pool using AWS APIs rather than the OAuth flows.

For client credentials/auth code grants - these are configured through the Hosted UI settings. You can in fact disable USER_SRP_AUTH and USER_PASSWORD_AUTH if you are only using the OAuth flows.