Where did you find that azure runs on linux?

I dont know of anywhere that Microsoft confirms, officially, that Azure, itself, is largely running on Linux. They share stats about what workloads others are running on it, but not, to my knowledge, about what it is composed of.

I suppose that would be an oversimplification, anyway.

But that Azure itself is running mostly on Linux is an open secret among folks who spend time chatting with engineers who have worked on the framework of the Azure cloud.

When I have chatted with them, Azure cloud engineers have displayed huge amouts of Linux experience while they sometimes needed to "phone a friend" to answer Windows server edition questions.

For a variety of reasons related to how much longer people have been scaling Linux clusters, than Windows servers, this isn't particularly shocking.

Edit: To confirm what others have mentioned, inferring from chatting with MS staff suggests, more specifically, that Azure, itself, is mostly Linux OS running on a Hyper-V virtualization later.