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AAD can be used as a SAML IdP.

https://learn.microsoft.com/en-us/azure/active-directory/fun...

What's the source of data and truth for your SSO?

The SSO IdP itself.

OpenID Connect seems like the current popular flavour. SAML seems to be increasingly considered legacy.

Indeed legacy, but you know how Fortune 500 companies are about new technology not directly relevant to their line of business.

Also, SAML as a spec is really complex precisely because it was created to satisfy a broad range of Enterprise-y requirements. I don't know if OpenID Connect is there yet. It certainly could be, the underlying spec (oauth2) could support a lot of variant complexity, and OIDC supports mobile and there are lot of extensions available or in progress. https://openid.net/developers/specs/