You're probably joking, but OSes like damn small Linux, knoppix, and puppy Linux can be configured to boot to ram. They're often used as a rescue os

I'm not joking. I used to be an Alpine user, but I ran it in disk mode. Any OS can probably be made to boot to RAM. You just need to get the initramfs to extract a rootfs image to RAM (tmpfs), and set that filesystem as root before handing off to init. What makes Puppy special is the stuff it does to persist changes.