coreboot doesn’t try to mandate how the boot process should look, it merely does hardware init and then passes on control to another piece of software that we carry along in firmware storage, the payload.
There is various software in that space that is either explicitly written as payload or can be made to work as one.
SeaBIOS is an open source implementation of the PCBIOS API that exists since the original IBM PC and was extended since. While originally written for emulators such as QEMU, it can be made to work as a coreboot payload and all the necessary code is in SeaBIOS’ mainline code.
Tianocore is the open source reference implementation of the UEFI Specifications that modern firmware for PCs is based on. There were various projects in the past to make it suitable as a coreboot payload, but these days this function is available directly in the CorebootPayloadPkg part of its source tree.
GRUB2 was originally written as a bootloader and that’s its most popular purpose, but it can also be compiled as a coreboot payload.
There are several projects using Linux as a payload (which was the configuration that gave coreboot its original name, LinuxBIOS). That kernel is often rather small and serves to load a current kernel from somewhere, e.g. disk or network, and run that through the kexec mechanism.
Two aspects emphasized by proponents of Linux-as-a-payload are the availability of well-tested, battle-hardened drivers (as compared to firmware project drivers that often reinvent the wheel) and the ability to define boot policy with familiar tools, no matter if those are shell scripts or compiled userland programs written in C, Go or other programming languages.
Heads is a distribution that bundles coreboot, Linux, busybox and custom tools to provide reproducible ROMs. Heads aims to provide a secure and flexible boot environment for laptops and servers. It supports features like measured boot, kexec, GPG, OTP, TLS, firmware updates, but only works on a limited amount of mainboards. For more details have a look at heads-wiki.