A few years ago I needed to add a new arch into Debian, it's a painful process, there are lots of arcane packages used to truly build and bootstrap the debian and they're all cross-dependent on each other. I wish there is a way like "linuxfromscratch" with clear steps to build Debian from source from scratch.

Yes you can bootstrap a Debian root filesystem for a different arch, but that's after someone already did the work before you can do that, which again, is really complex and I have there are better ways to do it now(e.g. adding a new CPU family to Debian)