What does the Manager field in Active Directory\Exchange and 365 actually do?

In Office 365 Managers get Notifications of User deletions as well as access to the users OneDrive

When an office 365 account is deleted or the Ad Sync Item is removed ( Either by deleting the object or removing the AD Sync group from the user ) it has 30 days before it gets deleted ( Emails and Files )

The manager get notified of this at 30 days , then again at 7 days

https://docs.microsoft.com/en-us/onedrive/retention-and-deletion

Creation of a Team Calendar

Outlook will automatically add Team Members ( who have the same Manager ) calendars to each outlook, as well as the managers

Expand : User Configuration – Policies – Administrative Templates – Microsoft Outlook 2010 or/and 2013 or 2016– Outlook Options – Preferences – Calendar Options – Schedule View

And Set policy Prevent Reporting Line Group Calendar from appearing .