For the sake of argument, let's presume that there are the following Stack Exchange Q&A sites (any resemblance to existing SE sites is coincidental):
Automotive Engineering — A Q&A site dedicated to vehicle engineering, incorporating elements of mechanical, electrical, electronic, software and safety engineering as applied to the design, manufacture and operation of motorcycles, automobiles, buses and trucks and their respective engineering subsystems.
Auto Mechanics — A Q&A site dedicated to helping automotive mechanics diagnose and repair vehicles, and to ask about business aspects specific to the automotive mechanic industry.
Cars — A Q&A site dedicated to discussing the performance and features of automobiles.
The Big Question
Let's say that our hypothetical OP wants to modify his DeLorean DMC-12 to add a custom part. This part will require more power than the automobile battery and electrical wiring harness is currently able to supply to the part. He now comes to the SE network with this 'big-picture' question in mind. So which of our hypothetical sites should he ask this on? None of them.
How To Ask the Big Question
His current question is too broad and not entirely on topic anywhere. So what this OP will have to do is break his question down into a series of questions and ask them in multiple locations. Some of his questions may not be welcome on any of the sites so he may need to do some research to answer those on his own. Here is how our wise OP will go about asking his 'big question':
On Automotive Engineering.SE, the OP will ask the following:
Note that the OP already did the work of designing a prototype and is now asking the community to objectively determine whether the design is capable of safely delivering the power needed for the custom part.
On Auto Mechanics.SE, the OP will ask the following:
- How do I securely mount this custom part between the front seats of a DeLorean DMC-12?
- This modified wiring harness doesn't fit behind the driver's side rear quarter panel on a DeLorean DMC-12, how do I get it to install without having to modify the entire harness?
Note that the OP did not ask how to install the entire part and wiring harness in one question, he attempted to do/understand it himself and then asked only about the specific areas where he had trouble.
On Cars.SE, the OP will ask the following:
Here are some remaining questions that the OP tried to ask but they were closed as off topic on the various sites:
How to install a wiring harness on a DeLorean DMC-12? (The closest fit was Auto Mechanics.SE, but they told the OP he needed to learn this on his own as it was too broad and showed no initial research effort. The site does not exist to teach you how to be a mechanic, but to help mechanics with actual issues they face. The OP ended up finding a book on this and learned, and then only asked about a specific portion of the install where he was having trouble).
What battery will supply 1.21 GW to a custom part? (This was closed as 'searching for a product recommendation'. This type of question is not well-suited to most sites on the SE network because answers are often too localized, among other reasons).
How to obtain plutonium? (No one was able to help the OP find plutonium and this question was closed as too localized, although he received numerous comments about Benghazi and Libyan terrorists).
The big question itself (This was promptly closed as 'too broad' everywhere the OP asked it and most of it was off topic on the various sites anyways).1
Did the OP's Big Question Get Answered?
In the long run, through the OP's continued research efforts combined with assistance from the various SE communities, the OP managed to accomplish his objective. But he wasn't able to come to any one site with this 'big picture' question and get all of it answered in one place. This was somewhat frustrating in the learning process, but in the end the OP learned to appreciate site scope and that this limitation allowed communities of experts to form around the specific aspects of automobiles that they know well (because automotives is a big field).
In summary, you may only be able to ask a part of your question here, and you will need to ask other parts elsewhere. Some parts may not be welcome anywhere, in which case you will have to conduct independent research to find your 'big answer.' This 'specialization' allows SE communities to gather around specific aspects of topics of interest within their respective fields and is common to the entire network.
1 I actually greatly simplified this analogy because this post is already too long. The original version specifically mentioned the design of the 'flux capacitor' (custom part), had related questions for Physics.SE, a question for Skeptics.SE about the location of plutonium globally, a spelling question about 'jigawatt' vs. 'gigawatt' for English.SE, and an ethical question about time travel for Philosophy.SE.
The majority of this answer is from a meta post I wrote for a specific site that I figured I'd share what I think are helpful parts of with the broader community.