You will see the usage in the debug logs of vbo365:

%ProgramData%\Veeam\Backup365\Logs

The usage of this accounts is not visible in the gui.

And you can see it in azure AZ „sign in log“ page.

Configuration of the accounts in vbo365:

https://helpcenter.veeam.com/docs/vbo365/guide/adding_backup_accounts.html?ver=50

In that folder, in the Veeam.Archiver.Proxy file, I can see the application ID’s only when the job started:

9/2/2021 2:28:27 PM   61 (8476) Initializing auxiliary backup accounts from configuration database...
9/2/2021 2:28:27 PM   61 (8476)  Application ID: 59dac531...
9/2/2021 2:28:27 PM   61 (8476)  Application ID: 6933a39a...
9/2/2021 2:28:27 PM   61 (8476)  Application ID: bb38b0da...
9/2/2021 2:28:27 PM   61 (8476)  Application ID: 170e0769...
9/2/2021 2:28:27 PM   61 (8476)  Application ID: 9542c5e2...
9/2/2021 2:28:27 PM   61 (8476)  Application ID: 6bf9b9f3...
9/2/2021 2:28:27 PM   61 (8476)  Application ID: 3d1c621f...
9/2/2021 2:28:27 PM   61 (8476)  Application ID: 22c6d51d...
9/2/2021 2:28:27 PM   61 (8476)  Application ID: ba13e15c...
9/2/2021 2:28:27 PM   61 (8476)  Application ID: 305b92bd…

In the Azure AD Sign-in logs, there are connections only in the option "Service principals sign-ins" but seems only one connection per day, and for Microsoft Graph and Office 365 SharePoint Online, there are no connection for Office 365 Exchange Online, actually there are no permissions for Office 365 Exchange Online, is that normal?

This backups accounts are only used for sharepoint sites (and one drive).

They are not used for exchange online backup Jobs.