After the POST is successfully completed the CPU searches Storage devices for special files that indicate the beginning of the OS and then loads it into memory.
The sequence in which the storage devices are searched is part of the configuration set-up stored in BIOS.
Once the Operating System (OS) is located, the CPU copies what’s called the 'boot record' from the OS into DRAM.
Control is then passed to the 'boot record' in DRAM, which then continues loading the rest of the OS.
This process continues until the OS load is completed.
When the OS load is completed the Desktop appears in the display and waits for you, the user, to tell it what you want it to do.