You can open any c2addon file as it's just a renamed .zip archive and see the plain javascript files therein.
Unless you already have a fairly good grasp of the Construct 2 SDK and also fair knowledge of Babylon.js I wouldn't immediately get into developing your own plugin that's supposed to work within the Baby 3D plugin suite. It's far from a novice task.
However you can of course study the plugins and behaviors provided and see how things are set up and ultimately learn how to change and expand things yourself. No doubt.
Just please don't expect me to provide detailed guidance in such matters.