This is a list of resources that I have been gathering so far to learn about the CRuby internals.
Ruby Hack Challenge is a repository on Github created by Ko1 (Koichi Sasada). In this repo, there are resources and documentation about CRuby development.
In his blog, Peter Zhu shares a lot of information about CRuby internals. You can find info about the CRuby GC, a guide about the CRuby code base, and more.
In his blog, Kevin Newton shares a lot of information about CRuby internals. You can find info about Prism, the new Ruby parser, a series about YARV (the CRuby VM), and a lot more.