Core concept
User Pools support email/phone signup, social login (Google/Apple/Facebook), SAML, OIDC. MFA + custom flows.
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How it works
Limitations: extending built-in flows painful. Custom email templates limited. Some prefer Auth0 or self-hosted.
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Trade-offs + gotchas
Consumer apps needing quick auth. Serverless apps needing AWS credentials for users (S3/DynamoDB access).