What environment is loaded when you select an OS?

Question

Iron

I would like to know what software environment is loaded when choosing an OS, for example, if both Windows and Linux are installed on the computer, in fact this is not a BIOS and the operating system has not yet been launched, then what is it?

Answer 1

Loader.
Here is a real example of GRUB
AND in Russian

Answer 2

bootloader, however.
Loaders are many, very different. True, except for the standard Windows and GRUB, it is rarely used, because it is difficult to configure them, and those two are also not easy :)