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.
