The Brunch framework purpose is to create a generic x86_64 ChromeOS image from an official recovery image. To do so, it uses a 1GB ROOTC partition (containing a custom kernel, an initramfs, the swtpm binaries, userspace patches and config files) and a specific EFI partition to boot from it.
WARNING to Windows users: the installation instructions require that you install Notepad++. I stopped reading at that point, unclear what might be involved afterwards… 🙂