That all depends on what platform(s) you're using. For example, IBM still gives you zOS, AIX, the System i OS, and so on. And there are other operating systems for other platforms.
The computing world doesn't begin and end with Wintel, you know; it may seem that way sometimes, though.
I would love to see something like https://pharo.org/ booting directly. Its an OS for all intents and purposes (To my realization, languages like Smalltalk, Lisps, Forths ARE Operating Systems)