I thought that maybe the kernel would be a binary, so it would not have to be recompiled like how I would assume it usually does.
There are options for binary kernels, just so you are aware.
As for documentation related to configuration of portage and /etc/portage/make.conf, I would, of course, be remiss to not point you to the Gentoo Handbook. The wiki contributors do really great work and the community is generally quite welcoming and open to implementation-specific questions.