Hello,
To get all events from any user's calendars you can use Client Credential flow (Application Permissions) which gives App only token and use it to query all calendars using below call.
https://graph.microsoft.com/v1.0/users/{userID}/calendars
and then use the calendarid's and pull the events by using the below call for each calendar_id.
https://graph.microsoft.com/v1.0/users/{userId}/calendars/{calendarID}/events
Currently, there is no direct call to get all events from all the calendars as of now. There is already a feature request raised in the Microsoft Graph Feature Request Forum, please upvote it so that it may be implemented in the future.
Hope this helps.
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