The bootloader is called freeldr
Hal is needed to run the bootloader
The kernel is called ntoskrnl.exe
SMSS.EXE is the first process run by ntoskrnl.

#from https://reactos.org/wiki/FreeLoader
Properly install a MultiBoot-compliant boot loader (for example, GRUB),
and configure it to load the file "FREELDR.SYS" as a Multiboot kernel (cause FreeLoader supports that).
See Boot FreeLoader from GRUB.
You cannot directly load ntoskrnl.exe as a multiboot kernel,
because it needs boot-time information that is extracted from the registry
and other specific stuff that the multiboot standard,
as a general standard, doesn't support.
Then, of course, you need to write the file FREELDR.INI according to your needs.


https://github.com/reactos/reactos/blob/master/boot/freeldr/FREELDR.INI