What do you define as low-level stuff? It seems that Node.js was used to communicate with the components running on a regular Linux OS, not really the low-level stuff that you would be able to do with, say, C.

Right. I'm not very familiar with Arduino but I assume its microcontroller was programmed in C or assembly. Probably it listens for simple commands such as "activate/stop left wheel", "activate/stop right wheel".

edit: Apparently, Arduino uses its own programming language: "Arduino hardware is programmed using a Wiring-based language (syntax and libraries), similar to C++ with some slight simplifications and modifications, and a Processing-based integrated development environment." https://en.wikipedia.org/wiki/Arduino