I suspect there exists some internal documentation at Ericsson that just hasn't been cleaned up and added to the source repository - but it's entirely possible

I suspect there exists some internal documentation at Ericsson that just hasn't been cleaned up and added to the source repository - but it's entirely possible that the core Erlang/OTP team solely relies on passing the knowledge on from engineer to engineer. I'm also not saying that the C code is unmaintainable. It's definitely a bear to dive into, but by spending enough time with it, it starts to unfold in front of you. The main issue I have, is that none of the specification/design documentation exists as part of the source repository. Maybe it doesn't exist at all, but in that case, I'd really hope that some of those core engineers would have taken the time to write some of that stuff down. In any case, none of it is readily available AFAIK.