You can use the administrator status to get the basic information and emails of other users, it is important to note that you need to grant the appropriate permissions, but you cannot get the chats.
For user information, please refer to this document. Here are my test results.
When you get another user's mailbox, you need that user to grant you access or share the mailbox with you.
More information about to get another user's mailbox:
https://learn.microsoft.com/en-us/graph/api/message-get?view=graph-rest-1.0&tabs=http
https://learn.microsoft.com/en-us/graph/outlook-share-messages-folders
Hope this helps.
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