Microsoft uses a Windows derived host OS running Hyper-V for the VMs.

https://techcommunity.microsoft.com/t5/windows-os-platform-b...

so the "real" azure nodes are some kind of special Windows+HyperV but for example their accelerator cards are linuxes: https://cfp.all-systems-go.io/media/all-systems-go-2023/subm...

We use Azure AppService. If you choose AppService Windows, it provides a virtual instance of Windows with IIS. If you choose AppService Linux, it provides a virtual instance of Debian (if I remember correctly). I'm not sure if Linux is running on top of Windows or Windows on top of Linux, but you get what you chose.

I'm assuming Microsoft has pivoted to SaaS and the Cloud, and the OS plays a lesser role in the current form of Microsoft. Gone are the days of Steve Ballmer. Windows and Office are no longer the primary cash cows.

I was there three years ago. Most (possibly all) of what was running on bare metal was Windows/Hyper-V. Some Azure service backends ran on Linux VMs; AKS backend for instance was on Linux I believe. Most were Windows/C# though. I know there was some initiative to allow for Linux bare metal for some internal services, I think to allow for running k8s clusters without the extra hypervisor layer(?), but it was still WIP when I left.

The Azure infrastructure, as I assume most hyperscale infrastructures are, is hybrid depending on the service. Service owners typically have a fair amount of liberty in deciding how they architect their service and what they use underneath it.

One interpretation of your question is the Azure API / RM engine itself, which if I understand correctly is written in C# and can run on either linux or windows or probably some containerized app engine version of either or both