I was diving into usage of the UEFI Shell today for some hour on creating a manual entry for Windows on a separate disk like so:
One of this times the holy archwiki is biting my brain very hard.. from firts reading i was not understanding only a little what this uefi shell part should be..
You can reach it from normal arch ISO it has an boot entry there.
running it will bring you to a prompt and from there you can run some commands the one i used was maps -r -b -b to let it stop when reaching the screen border.. as i have 19 entries for harddrive partitions plus it has extra entries in that line for usb and stuff.. the output was passing like the boot process only 100 times faster so only saw the last lines..
esp/loader/entries/windows.conf
title Windows
efi /shellx64.efi
options -nointerrupt -nomap -noversion HD0b:EFI\Microsoft\Boot\Bootmgfw.efi
only to find the correct FS alias for the drive with windows on it.. HD0b in the example ..
mine is FS1 or BLK6 but HD1b was not working.. with this i can boot windows on extra drive again .. but i do not think there is a simple way to use this process to create a working systemd-boot entry for windows inside installer process ..