The OS runs on hardware, what do you think is below the OS you're running?
Even excluding VMs, there are a lot of OSes running in a modern computer. Some chips have their own. BIOSes and Secure Enclave, networking chips, and probably a dozen more.
Try to understand the context of more than one comment at a time if you can. This person said "it's turtles all the way down" because there are two layers, the browser and the OS, which is ridiculous.
Even excluding VMs, there are a lot of OSes running in a modern computer. Some chips have their own. BIOSes and Secure Enclave, networking chips, and probably a dozen more.