I know how Windows Boot Manager (aka WBM, BOOTMGR) works. :P
EFI or BIOS loads WBM, which resides in the EFI system partition or active partition (also tagged "System") as applicable, which then reads the Boot Configuration Data (aka BCD) also located in that partition. Windows itself is located in a separate partition from WBM and BCD.[1]
WBM then boots a specified Windows installation from a given partition, which is tagged as "Boot". If desired, WBM can also chainload into another bootloader (eg: NTLDR, Windows 9x bootloader, GRUB, etc.) to boot something other than Windows NT6 and up.[2]
[1]: https://en.wikipedia.org/wiki/Windows_Boot_Manager
[2]: https://en.wikipedia.org/wiki/Booting_process_of_Windows_NT#...
The code in the PBR (Partition Boot Record) chainloads a file named BOOTMGR (which is a boot manager) that looks at what to boot in a file residing in the \boot folder called BCD (Boot Configuration Data).
Q:How do you call the partition (volume) where this stuff is?
A:System