Auth is zero value-add table-stakes requirement with a lot of complexity. I haven't a seen a product with enough built-in. External provider is the way to go.
Depends on features and budget. AWS Cognito is very cheap, but not many features out of the box. Google Cloud Identity Platform seems to be same category as Cognito with some differences. Auth0 is a little more expensive, but has a lot of features. There are others like Ory, KeyCloak, etc.