@Avital Malka, Thanks for posting in Q&A.
For the operatingSystem, I find it is an attribute for the managed devices. In my environment, for windows devices, the value is "windows". For iphone, it will show "iOS". For android device, it will show "Android".
https://learn.microsoft.com/en-us/graph/api/intune-devices-manageddevice-list?view=graph-rest-1.0
Hope it can help.
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