Hello there,
I suppose this ADSS service has been integrated with ADSync service .
Azure AD Connect installs an on-premises service which orchestrates synchronization between Active Directory and Azure Active Directory.
Azure AD Connect sync: Understand and customize synchronization https://learn.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-sync-whatis
ADSync service account https://learn.microsoft.com/en-us/azure/active-directory/hybrid/concept-adsync-service-account
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