A unified kernel is one that includes the initrd (low effort explanation). Dracut makes it with the correcet arguments and places it directly onto /boot/efi/EFI/Linux.
That's it. With systemd-boot picking up and offering to load anything from Linux, you don't need anything else (loader menus etc).
sudo dracut -fvM --uefi --hostonly-cmdline --kernel-cmdline "root=UUID=b6b8fa59-92cc-4d03-8d8f-d66dab76d433 ro rootflags=subvol=root resume=UUID=fb661671-97dc-45db-b720-062acdcf095e rhgb quiet mitigations=off"
The part after --kernel-cmdline is just the command line (kernel options).
Find cmdline at /proc/cmdline or /etc/kernel/cmdline. See above for what is needed (and what not).
Add --kver for a different kernel version. The number following this is what appears in /lib/modules.
E.g
otheos@kepler ~]$ ls /lib/modules/
5.18.16-200.fc36.x86_64 5.18.5-200.fc36.x86_64 5.18.6-200.fc36.x86_64
So if you want to build for a different kernel, you add, e.g. --kver 5.18.5-200.fc36.x86_64.
This littke script should take care of it:
#!/bin/bash
newkern=$(ls -t -1 /lib/modules | head -1)
echo $newkern
cmdline=$(cat /proc/cmdline)
echo $cmdline
sleep 5
sudo dracut -fvM --uefi --hostonly-cmdline --kernel-cmdline "$cmdline" --kver $newkern
Needs to build a new universal kernel after every kernel upgrade. Or make the above script run after each kernel upgrade to "automate the process".