This thread seems to have wandered off-topic and degraded into argument rather than enlightenment. Here is a re-print of most of the original question:
If I sat down with a pen and paper and nothing else, how would I go about working out what was needed and how it all works to build...
Belaying for a moment a consideration of the dimensions and depth of "nothing else," there have been a few good comments that all seem to boil down to this: learn the fundamentals first.
You need tools to design anything, including the design and manufacture of more advanced tools. A blank sheet of paper (often a paper napkin or the back of an envelope) is they way I begin any design. But I don't use "nothing else". Instead I use more than sixty years of education, practice, and experience to put meaningful marks on the paper. At some point I will trade a napkin sketch for a real engineering drawing, sometimes constructed with the help of computer-aided design software.
In the end, I will probably make a prototype, using any of various construction techniques picked up along the way. But I started this journey small. Reading biographies of Edison, Franklin, Faraday and Bell inspired the younger me. Reading and doing experiments described in
The Boy Electrician encouraged me to learn more. Having supportive parents and grandparents was a huge help, but I sold Christmas cards; washed and waxed cars in customer's drive ways; delivered newspapers; cut lawns and performed various other small tasks that grownups would entrust to a youngster to earn enough money to purchase electronic test equipment and (sometimes) electronic parts. My first purchase was an
RCA vacuum-tube voltmeter (VTVM) in kit form.
In those early days, I visited the alleys behind Radio and TV Repair Shops to salvage complete chassis of parts from radios, record players, and televisions that cost more to repair than it cost to replace with new. (The same is true today of course, except there are no repair shops left from which to salvage parts.) There were also discarded vacuum tubes that often had some life left in them, although they could not meet the same performance specifications that "as new" versions did. All this "stuff" made for a great set of learning tools in the 1950s and early 1960s.
Today there is so much inexpensive electrical and electronic "stuff" available to "play" with, there is virtually no reason not to learn electronics if you just add a little ambition and interest. But you
must start by learning the fundamentals, the basics. I am not saying you must perform the same experiments the Greeks did with amber and dust bunnies to learn about "
elektrons" but at least read about it and learn where we came from. The human race has built on the shoulders of giants throughout recorded history. There is no need to re-invent
all the wheels in order to learn how to use what is available.