Frida's core is written in C and injects Google's V8 engine into the target processes, where your JS gets executed with full access to memory, hooking functions and even calling native functions inside the process. There's a bi-directional communication channel that is used to talk between your app (Python?) and the JS running inside the target process.

On top of this C core there are multiple language bindings (Python, .NET and a browser plugin), and it is very easy to build further bindings for other languages and environments (Node.js could be a future binding if anyone's interested in helping out with that).