The following link might be helpful:
How objects and credentials are synchronized in an Azure AD Domain Services managed domain
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
(Please don't forget to mark helpful replies as answer)
As far as I know Office 365 groups and Exchange distribution lists are synchronised to Azure ADDS.
Office 365 uses Azure Active Directory (Azure AD) to manage user identities behind the scenes, by default all users and groups from an Azure AD directory are synchronized to an Azure AD DS managed domain.