Operating Systems, what do they do:

The operating system on a computer handles essentially everything the computer does.  It manages the "peripherals":  printers, monitors, mouse, that stuff.  It handles the file system.  It handles all processes running on the machine and communication between these processes.  It is essentially impossible to write any sort of program without making use of the operating system.

For example:  what is the one thing the operating system does for you that is so invisible, you're not aware the operating system does it?  Sooner or later, every program must terminate, and THAT requires an operating system.

Memory management does not usually use a specific operating system command; you usually use an assembler command to indicate what you want to store and where BUT nearly every machine uses virtual memory and that is handled by the operating system.

The very act of running, means that your program is being managed on a process tree, and that is handled by the operating system.  If you are running, some other program started you running, and that program had to use an operating system command to do that!

Physical memory:  you have semiconductors that can store bits.  You have a finite number of them, but usually a large finite number.

Virtual memory:  your computer "imagines" that you have more space than you really do.  you can read from it and write to it, and the operating system will store in physical memory until you run it, and then starts moving data around to the disk to free up more physical memory, and this happens so invisibly you don't even notice it.