How to build the official style iso image?

What is the process used to build the official iso images?

I found live-server.yaml, but the official
images seem to have some additional boot loader (which also allows to edit the kernel boot options on boot time.

You might be able to use the link shown:
url:Index of /releases/ (clearlinux.org)
to see how previous and current releases are put together.

You mean the boot timeout?
Follow the rabbit trail: live-server.yaml → live-server-post-install.sh → wait-to-boot-post.sh