Every Azure AD directory comes with an initial domain name in the form of domainname.onmicrosoft.com.
The initial domain name cannot be changed or deleted, but you can add your corporate domain name to Azure AD as well. For example, your organization probably has other domain names used to do business and users who sign in using your corporate domain name. Adding custom domain names to Azure AD allows you to assign user names in the directory that are familiar to your users, such as '[email protected].' instead of 'alice@domain name.onmicrosoft.com'.
Scenario:
Network Infrastructure: Each office has a local data center that contains all the servers for that office. Each office has a dedicated connection to the Internet.
Humongous Insurance has a single-domain Active Directory forest named humongousinsurance.com Planned Azure AD Infrastructure: The on-premises Active Directory domain will be synchronized to Azure AD.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/fundamentals/add-custom-domain