Set aside (for the moment) PXE and Wimboot.
For BIOS the files involved are:
1) \boot\boot.sdi <- this is only small NTFS filesystem that is used to mount the boot.wim (and that normally will become drive X:\)
2) \sources\boot.wim <- this is the actual (imaged) filesystem of the PE (the contents of whch will normally later appear inside X:\)
3) \BOOTMGR <- this is the actual OS loader that uses a configuration file to give you booting choices
4) \boot\BCD <- this is the configuration file for BIOS
For UEFI the booting files involved are:
1) \boot\boot.sdi <- this is only small NTFS filesystem that is used to mount the boot.wim (and that normally will become drive X:\)
2) \sources\boot.wim <- this is the actual (imaged) filesystem of the PE (the contents of whch will normally later appear inside X:\)
3) BOOTMGR.EFI (actually \efi\boot\bootx64.efi or \efi\boot\bootia32.efi) <- this is the actual OS loader that uses a configuration file to give you booting choices[1]
4) \efi\microsoft\boot\BCD <- this is the configuration file for UEFI [2]
[1] As a matter of fact the \efi\microsoft\ folder is somehow a "forced" addition by the good MS guys, what the UEFI specifications say (among many other things, they are 2200 pages of mostly senseless crap) is that UEFI should look for a file called bootx64.efi (if 64 bit hardware) or bootia32.efi in \efi\boot\ (not \efi\microsoft\boot\) and the BOOTMGR.EFI you find on DVD is a copy of either the bootx64.efi or of the bootia32.efi.
[2] please note how the contents of \boot\BCD and of \efi\microsoft\boot\BCD are NOT necessarily the same, in the case of BIOS the BCD will have an entry pointing to WINLOAD.EXE (which is the actual windows loader for BIOS) whilst in the case of UEFI the corresponding entry will point to WINLOAD.EFI, and also take note that the previously mentioned boot.sdi and boot.wim are only the "default", a BCD entry may well be pointing to (say) \Mickey\mouse.sdi and to \Uncle\Scrooge.wim
I know it seems complex (mainly because it is complex ) but you are still taking it IMHO from the "wrong" angle, read first the TinyPxe guide, there is a specific page for iPxE wimboot booting of windows (7,8 and 10 use the same approach):
http://mistyrebootfi...npe_wimboot.htm
where the actual "virtual mapping" that wimboot does transparently is also explained, then, if you have questions ask them, but you should first become familiar with the processes involved, as opposed to do random tests somehow attemptng to reverse engineer the Dell examples.
Wonko