It worked by building the system into the initramfs, then using a kernel feature to basically bake the initramfs into the kernel image, and then using efistub to make the kernel bootable
Kernel docs > admin-guide/efistub: https://docs.kernel.org/admin-guide/efi-stub.html :
> Like most boot loaders, the EFI stub allows the user to specify multiple initrd files using the “initrd=” option. This is the only EFI stub-specific command line parameter, everything else is passed to the kernel when it boots
Arch Wiki > EFISTUB: https://wiki.archlinux.org/title/EFISTUB :
> The option is enabled by default on Arch Linux kernels, or if compiling the kernel one can activate it by setting CONFIG_EFI_STUB=y in the Kernel configuration.
eaut/efistub is a script for managing signed efistubs: https://github.com/eaut/efistub
... Systemrescuecd can be PXE booted (netbooted) from tftpd managed by Cobbler or Foreman.
From "Booting Linux off of Google Drive" https://news.ycombinator.com/item?id=40853770#40859485 re: signing and iPXE
Ventoy boots via grub/efi but there are blobs: https://news.ycombinator.com/item?id=40619822#40620279
"Ask HN: Where can I find a primer on how computers boot?" https://news.ycombinator.com/item?id=35230852
systemd-boot and grub support booting to efistubs
"Migrate from systemd-boot to EFISTUB" https://www.reddit.com/r/archlinux/comments/14jt8yv/migrate_... :
> You don't "disable" systemd-boot, you just don't run it. Or you can delete the boot entry for it with efibootmgr if you want. You also don't need to "set up" EFISTUB, the stub is already built into the Arch kernels, so they're already bootable. So all you need to do is make a boot entry in your UEFI using efibootmgr with the correct kernel parameters, most likely the same ones you used for systemd-boot.
Fedora > Changes/Unified_Kernel_Support_Phase_2 > Switch_an_existing_install_to_use_UKIs: https://fedoraproject.org/wiki/Changes/Unified_Kernel_Suppor... :
dnf install virt-firmware uki-direct kernel-uki-virt
kernel-bootcfg --show
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"
Gentoo wiki > EFI_stub https://wiki.gentoo.org/wiki/EFI_stub :
> It is recommended to always have a backup kernel. If a bootmanager like grub is already installed, it should not be uninstalled, because grub can boot a stub kernel just like a normal kernel. A second possibility is to work with an additional UEFI entry. Before installing a new kernel, the current one can be copied from /efi/EFI/example/ to /efi/EFI/backup. [And then efibootmgr]