Like you, I get modular patching, and I don’t understand how to code on the Arduino at all. I am a noob only slightly less noo than you. BUT I have managed to install programs into my nano’s Arduino board. It took some trial and error (and was frustrating at times), but it was worth it.
The idea is that the Arduino chip can extend the functionality of your nano by doing various additional kinds of DSP (digital signal processing). What the pins on the left side of the board do depends on the code.
Usually detailed instructions will be provided in the “readme” on the github repository and also in the first part of the program (or “sketch” as they are called in Arduino). Keep a copy of those in front of you when you are using a given sketch.
Rather than trying many different kinds of sketch, you might want to pick one and explore what you can do with it.
Detailed instructions for installing sketches are in this thread http://www.sound-machines.it/forums/topic/what-if-i-want-the-original-firmware-back/ . I am the noob who says “OMG OMG OMG” a lot on that thread. Niklas gives great detailed advice, and Matthew Freidrichs uses the Arduino to provide something I wish the nano had: a modulatable LFO.
That’s the whole point of the Arduino: it can be coded to do things that the original synth can’t do. The sky is the limit. But since each sketch can use the pins in a completely different way, you have to pay close attention to the comments and instructions on the sketch you are using.
The other general comment I want to make (which is not very clearly spelled out in the documentation I have seen) is that the code available to the Arduino can be extended by adding various libraries to the IDE on your computer. Some sketches, perhaps many, rely on those additional libraries, and if you haven’t installed those libraries on your PC, Arduino will complain when you try to compile and then upload (which means ‘load into your nano’) the sketch.
Niklas’ instructions will help you add the necessary libraries.
With all that said, if you are happy with the “hard” modules on the nano, there is no obligation to use the Arduino side of the board at all. Just do what you enjoy. There is so much to explore – nobody could cover all of it!