Basically, Okta generates a JWT after authentication and passes to B2C. B2C copies the JWT but signs it with the B2C private key rather than the Okta one. In addition, B2C modifies the JWT according to the policy.
Yes - you can provide ROPC endpoint. But the flow is not secure hence deprecated in the new OAuth spec.
B2C doesn't support client credential flow but it's in the pipeline.