Enterprise SSO federation

Design enterprise SSO for a SaaS that must federate with thousands of customer identity providers (a mix of SAML and OIDC) and keep user lifecycle in sync, when HR offboards an employee in the customer's IdP, that user must lose access on your side within minutes, not at next login. Cover the login flow, how you map an external identity to your internal account without collisions across customers, JIT provisioning vs SCIM for lifecycle, and how you handle a customer's IdP signing-cert rotation without breaking their login.