Hello
Thank you for your question and reaching out. I can understand you are having queries related to moving from On-Perm to Azure AD.
First you'll need an Azure Active Directory (Azure AD) tenant that is associated with your subscription, either synchronized with an on-premises directory or a cloud-only directory.
After that you can create the managed domain (Azure ADDS).
Then synchronize from your Azure AD -> Azure AD DS.
The steps above are all part of the prerequisites:
Tutorial: Create and configure an Azure Active Directory Domain Services managed domain
https://learn.microsoft.com/en-us/azure/active-directory-domain-services/tutorial-create-instance
References:
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