Outlook has a simplified Permissions dialog, which is used on folders in Exchange accounts in your profile. Shared mailboxes and subfolders use the old "classic" permissions dialog.
The simplified dialog makes it clear what the permissions do, but it lacks the ability to customize the permissions or give another user Owner permissions.
To create custom permissions or to give another user Owner permission, you need to use the classic permissions dialog.
This registry key was added to Office in version 1903 to bring back the classic permissions dialog.
HKCU\Software\Microsoft\Office\16.0\Outlook\Options\Calendar\ DWORD: ShowLegacySharingUX Value: 1
More Information
The Exchange administrator can set permissions on mailbox folders.
Add-MailboxFolderPermission (ExchangePowerShell) | Microsoft Docs
