Probably one of the biggest things you are gong to run into with your self education here is that you aren't dealing with industry standards, but a very proprietary Logitech in-house build. This means, that while you WILL learn stuff along the way, you won't actually be able to apply any of the proprietary information to other standards across the audiovisual industry. That is, speaker building talks in a language of math. Volume, ported/non-ported boxes, power, ohms, etc. They don't talk VGA. They don't really talk about circuit boards, but about voice coils. Most amplifiers are removed from speakers and are a different discussion in themselves. Crossovers are a solid week's worth of discussion I would imagine.
So, while you can easily make your own D-Sub-15 cable and pin it out however you may like, you first have to identify which pins go to what inside the proprietary product and that's going to be a fair bit of work for something that is possibly completely undocumented outside of their design labs.
Mostly, my guess, is that this is all about taking audio in/out of the main amplifier/distribution box (not to ever be called a subwoofer).