I've been following this thread and others for a little while and not sure of this will help or shed any light, but it appears to me that these devices have a secure boot similar to UEFI (windows 8,8.1, etc secure boot)and without the particular file in the root setup u won't ever get the bootloader, has anyone tried an Android emulator?
The reason I ask is because in 2016 some states passed a law that required cell phone makers to enable device protection to reduce cell phone theft, hints why earlier versions on Android the files could be found. But on any new
device that shipped with Android 6.0 or higher you can't remove FRP, so what I'm getting at is the file set up that "builds" the OS is different and it works more like a script instead of an exe file. Overall preventing access to root, bootloader and other exploits to flash a stolen phone.