There are brands like OnePlus, Sony, LG, etc. that make their kernel source code live for the developers so that new custom ROMs and tweaks can ve developed for the respective smartphone brand’s devices. These kernel source codes serve as a mechanism to boot into the ARM platform as soon as your boot into the primary bootloader. Operating systems like Microsoft Windows, utilize the Advanced Configuration and Power Interface (ACPI) for performing the same operation, while the low-level hardware initialization is handled by the Unified Extensible Firmware Interface (UEFI).
And if you are a power user, then it is now possible, via some small tweaks to port UEFI and ACPI support to existing ARM devices. This will mean that you can now boot over the Windows OS on Rasberry Pi devices. Note that Windows 10 natively support the ARM architecture which is basically a plus point for those who are in the porting game as it makes the process a bit easier. As per a developer, called Evsio0n, has shared proof of the concept via Twitter where you can see the method to boot Windows 10 on ARM on the Qualcomm Snapdragon 835-powered Samsung Galaxy S8.
Interestingly, the developer has shared the source code to build the barebone UEFI firmware which is helpful for the Samsung Galaxy S8 based on the TianoCore project.
— Evsio0n (@boby2001820) April 13, 2020
If this has gotten your intrigued, then you can grab the firmware from here but note that due to the lack of complete ACPI implementation, you will not be able to boot the Windows 10 entirely. Moreover, as per the XDA post, the memory management unit (MMU) is working to some extent, which is enough for Debian Linux with mainline Linux 5.x kernel to boot. This is a nice development, and we might be not too far away where we would be seeing a full-grown Windows 10 OS being ported over to Android devices.