bootmgr code is in the PBR
1.After pressing the power button, the PC’s firmware initiates a Power-On Self Test (POST) and loads firmware settings. This pre-boot process ends when a valid system disk is detected.
2.Firmware reads the master boot record (MBR), which locates the "active" partition and then transfers control to that partition's boot sector (PBR) which then starts Bootmgr.exe. Bootmgr.exe finds and starts the Windows loader (Winload.exe) from the Windows\System32 folder on the partition which contains the copy of Windows selected from the boot menu.
3.Essential drivers required to start the Windows kernel are loaded and the kernel starts to run, loading into memory the system registry hive and additional drivers that are marked as BOOT_START.
4.The kernel passes control to the session manager process (Smss.exe) which initializes the system session, and loads and starts the devices and drivers that are not marked BOOT_START.
5.Winlogon.exe starts, the user logon screen appears, the service control manager starts services, and any Group Policy scripts are run. When the user logs in, Windows creates a session for that user.
6.Explorer.exe starts, the system creates the desktop window manager (DWM) process, which initializes the desktop and displays it.