In just about every computer science textbook, a computer consists of just three blocks: the hardware, the operating system, and then all the (user-land) software that runs on the OS. In actual fact, unbeknownst to the user, almost every computer has multiple operating systems running at the same time, managing various different parts of the computer -- and worryingly, these OSes are usually proprietary, closed-source, bug-ridden, and have extensive, low-level access to your data.