Hi,
The following link might be helpful:
How objects and credentials are synchronized in an Azure AD Domain Services managed domain
https://learn.microsoft.com/en-us/azure/active-directory-domain-services/synchronization
You can check what isn't synchronized to Azure Active Directory Domain Services (Azure ADDS) over here:
What isn't synchronized to Azure AD DS
https://learn.microsoft.com/en-us/azure/active-directory-domain-services/synchronization#what-isnt-synchronized-to-azure-ad-ds
(Please don't forget to mark helpful replies as answer)
Best regards,
Leon