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If you are syncing your password hashes, then the synced accounts will use the on-premises Active Directory password policies. This does not carry over the password expiry policy as the Azure AD account passwords are set to never expire here however if you are forcing users to change passwords on-premises after xx days, then this will update their Azure AD password once the password is changed anyway.
You can also refer below article,
https://learn.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-password-hash-synchronization#password-complexity-policy
If you are looking for the Azure AD password to get expired when on-premises password expires, then you have to configure the Azure AD password policy which should be matching on-prem password expiry.
You have to refer above article to configure the Azure AD password policy.
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