The source for Erlang/OTP is 71.6% Erlang: https://github.com/erlang/otp - but yes, some of the lowest-level parts are written in C.

As for universality, not sure where you’re getting that. No one’s saying you should use Erlang for writing device drivers or OS kernels, or the lowest-level parts of the VM. Erlang is a medium- to high-level language, and should be evaluated as such.