Yes, AD connect is always one way sync for objects (users, groups, contacts and devices).
AD connect syncs objects from on-prem to Azure AD.
AD connect writebacks only few user attributes from Azure AD to on-prem when exchange hybrid is enabled.
Also, using AD connect you can writeback few object types like, groups and devices.
You will have to configure AD connect to enable writeback of users, groups and devices.
You can go through below articles to enable to same,
Plan your group writeback: https://learn.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-group-writeback-v2
Enabling group writeback: https://learn.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-group-writeback-enable
Note: Group writeback option is available only if you have Exchange in your on-premises instance of Active Directory.
To get more information on how AD connect works, you can refer below article,
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