What tools do Google, Amazon or Microsoft use?
They are running millions of VMs,
Presumably its proprietary?
Google uses kvm. They probably contribute as well although I haven't checked.
From a quick search Azure uses a private build of hyper-v which surprises me actually but it's possible.
Edit : millions of VMs doesn't mean anything, at the end of the day it's just servers each running a bunch of VM, which people have been doing for 20+ years. There is no scaling issue there, the hard part is storage, networking, orchestration, managed services, power...
[0]: https://techcommunity.microsoft.com/t5/windows-os-platform-b...
Regarding AWS, I would also add Firecracker for AWS Lambda, which uses KVM.
In my opinion Proxmox does not have tooling like that.
So if I had written my comment better it would be what tooling and fabric these giants use and to what extent they share the tools and upgrades back to open source projects.