This URL will return details about the logged-in user. The identity provider can either be the default or a custom identity provider configured in the BTP Trust Configuration. The response will differ for OpenID Connect and SAML protocol.
Update 10.03.2026: If you want to use an SF field, that is currently not synced, it must be added on the source system sync. First on the Properties tab on sf.user.attributes and then it can be used in the transformation of the source system (in this case SF).