The main function of a program called Bootstrap Loader is to find the operating system and the latest function of the BIOS. Bootstrap is code that is only half a dozen lines long at the end of the POST program.
When the POST Test completes its process, it transfers it to Bootstrap. Bootstrap then searches for the OS (eg Hard Disk, CD / DVD) as instructed in the BIOS setup. Once the boot sector contains information related to the OS, the rest of the process is transferred to the OS.