
If you use Microsoft 365 you will use Azure Active Directory anyways cause AAD is the "authentication backbone" behind the Microsoft Cloud Solutions. Depending on your organisation size and IT architecture administration can also be done through the MS365 Admin Center, but i its a bigger company and you want to enforce stuff link conditional access for example you will have to use Azure Active Directory.
Take a small read here:
https://learn.microsoft.com/en-us/azure/active-directory/fundamentals/active-directory-whatis
I hope this information helps you resolves the questions.
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