They used to use the nix-daemon to realise derivations but that too was replaced original software at some point I believe. Everything else has been guix own tooling for a long time.
They still use the same derivation file format (.drv) and the basic concepts are the same (“conceptual fork”), so you could theoretically be able to realise a derivation produced by guix using nix-daemon/nix-store but that’s about it as far as similarities go.
If you don’t believe me, go take a look at the source code. I promise you won’t find a single line of C++ in it, only a ton of parenthesis ;))))