A MBR (Master Boot Record) boot sector does not contain a specific "file" but rather a small piece of executable code called a "bootstrap loader" which is responsible for identifying the active partition on a hard drive and loading the corresponding operating system bootloader from that partition; essentially, it's the code that initiates the booting process by telling the system where to find the OS to load. 

two Dell ways: 

https://www.dell.com/support/kbdoc/en-us/000145519/how-to-create-a-bootable-usb-flash-drive-using-dell-diagnostic-deployment-package-dddp

https://www.dell.com/support/kbdoc/en-us/000136959/create-a-bootable-usb-flash-drive-using-the-diskpart-utility

This makes the USB drive bootable. When selected at startup, it boots straight into the Windows installation.

g:  ( "g" is DVD drive letter)
cd boot (changes to boot directory)
G:\boot bootsect /nt60 d:  ("d" is USB drive letter)

Bootsect.exe updates the master boot code for hard disk partitions to switch between Bootmgr and NT Loader (NTLDR).

Applies the master boot code that is compatible with NTLDR to <DriveLetter>. The operating system installed must be older than Windows Vista.

bootsect /nt60 E:

Applies the master boot code that is compatible with Bootmgr to <DriveLetter>. The operating system installed must be Windows 8, Windows Server 2012, or newer.

When installing Windows, the operating system environment used is called the Windows Preinstallation Environment (WinPE), which is a lightweight version of Windows designed specifically for deployment and troubleshooting, allowing you to prepare a computer before installing the full Windows operating system