This is probably the hypervisor that Azure VMs run on, or perhaps Windows Server. Unlike Linux, (most) legacy apps (if they target NT) don't need VMs or containers; they run natively. You can compile for Windows 2000 on Windows 11 25H2 and run it natively on the host to test.

Why do you need VMs or containers? Windows's architecture allows much granular control of applications, so it should be possible to limit what the applications can access to a great extent already. Unlike Linux the system ABI is stable so you can run older applications without shipping the entire userspace.

Also Windows 10 LTSC exist (not 11 with all the rounded modern UI BS). It shows how good Windows could be.

> with legacy apps etc run seamlessly in VMs or containers?

We kind of have examples of that already in DOSBox. Even where Windows OOTB compatibility fails, getting some ancient piece of software running in DOSBox is often not an issue.