From @Vasil Michev :
synchronization is one-way, from AD to Azure AD/Office 365. There are only few attributes that can be written back, and that's mostly for Hybrid configurations, and passwords if you have the corresponding feature (and licenses) enabled.
There is no built-in functionality that syncs users from Azure AD to on-premises AD. If that's what you are after, you can simply export the list of users via PowerShell (Get-MsolUser/Get-AzureADUser) or the Graph API, along with any relevant attributes, then use the exported data to recreate them in AD (again, PowerShell helps). You cannot export passwords. Once the export/import is done, you can "match" the on-premises users with the cloud ones and give them the SSO experience. The process is known as soft-match: https://support.microsoft.com/en-us/help/2641663/use-smtp-matching-to-match-on-premises-user-accounts-to-office-365
Hope this helps!