I use a Microsoft 365 cloud deployment and I have an organization where users are created. All of these users are hosted in Azure AD. I send emails in Exchange Online.
For collaboration, we use Teams and SharePoint. Basically, all of these Microsoft products are on Azure AD. This is due to the fact that for you to use any of these products, users have to be created and these users are being hosted in Azure Active Directory. Without the users in the first place, the products are not used.