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I had an exact need some time back, so the reason why the "Operating System" column in the list view of Azure VMs shows only the generic name of the operating system (e.g. "Windows") is because this is the default behavior of the Azure portal. The list view is designed to show a summary of the VM properties, and the detailed OS information is not included by default.
I just thought of this workaround, If you need to have the detailed OS information, you can customize it using tags, at the time of creation of VM and even for the existing VMs, you can add a tag with its OS type and add it as a column in the list view from Manage Vire->Edit columns.
Another workaround is to use PowerShell to get a list of VMs along with its OS details but will be a bit of work.
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