In general: the firmware of a device is limited in what it can do: the firmware itself is not a full operating system and instead is designed to do just enough to load up some code from a storage device.
That’s where the bootloader comes in: the bootloader is some code that can be run from within the firmware that gives instructions on how to load up a full operating system from some storage device.
A computer doesn’t really “know” how to do anything. It has to run a program, or multiple programs. When you power it up, it just goes to a specific memory address and executes whatever is there.
But the main program (the Operating System, such as Windows) is on a storage device such as a Hard Disc Drive or Solid State Drive. It isn’t directly accessible to the microprocessor. The microprocessor literally cannot run the OS at startup.
The computer’s microprocessor (its “brain”) doesn’t even know how to *talk* to the HDD/SSD. Or the mouse, or keyboard, or monitor. So it doesn’t know how to get the OS and run it, because it can’t communicate with the device that holds it.
So there’s a really small program stored on a chip in the computer, called the Binary Input/Output System (BIOS). The microprocessor *can* talk to it. The BIOS is a program knows how to do basic communications with some pieces of the computer. It talks to the HDD or SSD and gets the microprocessor to run a simple program from one of them called the Bootloader.
The microprocessor runs the Bootloader and it copies the OS into RAM (memory) and then tells the computer to run it. It’s called the bootloader because this process is whimsically described as the computer “pulling itself up by its bootstraps”.
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