Boot sequence. - what software takes over and runs your computer when you turn it on? The operating system, right? No. In that case how would you be able to explain dual-boot computers? Those are the ones (unusual) that contain two or more operating systems within them and, when turned on, can choose between them. Windows on Monday, linux on Tuesday. Then, it must be the software that makes or allows that choice, right? No. Do you have a computer made by Hewlett-Packard that prints the H-P company logo on the screen every time you turn it on, or by Dell that prints Dell's logo? That's the software that gets immediate control when you first turn the computer on. When it's done, it gives control to the software that chooses between operating systems. Once it has decided, then the chosen operating system takes over. They do whatever they are designed and configured to do and usually will eventually run a user interface (like a desktop, or a command shell).
Where are these three pieces of software stored, when the computer is off? The manufacturer advertiser program resides on the motherboard, inside one of its chips. The OS chooser lives at the beginning of some storage device, maybe a bootable DVD, bootable USB, or hard disk partition. The operating system also lives on some storage device, maybe the same one where the chooser lives or maybe a different one. As a matter of common vocabulary, the "manufacturer advertiser program" is called firmware code, the OS chooser is called the bootloader.