Any consumer electronics is difficult to get into. Certainly the auto industry. Railroads. It's difficult to start an ISP, even a non profit one. I think the list goes on.
I had to find this. This explains on an atomic level how semiconductors work. For me this was important for me to gain an intrinsic understanding of what is going on in computers.
I think a good answer that you will understand is too long for this format. I gave a brief answer but then I went off looking for better information. Sorry to offend you.
I think this course from Ben eater on how he built his own CPU from logic gates might explain a lot.
I think it also covers how transistors work which is fundamental to how gates work.
Flashing code to a chip doesn't really involve light.
you used switches on the front panel to load code into the computer by setting individual bits high or low. Typically you toggled in the bootstrap loader, which was a program that read a sequence of number directly into a spot in memory. The first program loaded by the bootstrap loader was usually the absolute loader. This was another program that loaded data from some peripheral, similar to the bootstrap loader, but it could do error checking and also load to non- sequential locations.
3-the Internet isn't light. It's electricity. On fiber the bits may be temporarily encoded as light, but overall it is electric.
4- You can understand it all if you want. It depends on the depth to which you want to understand it. You can understand a mouse has a plastic shell. You need some organic chemistry and chemical engineering to understand how to design plastic.
5- I recommend Ben Eaters YouTube channel to get a good overview of the basics.
Ive done it just for fun. It was some time ago. Bought a screen in a frame, a squeegee, tried a couple mask methods but was most pleased with the photosensitive stuff. It is quite fun and results were impressive for the short time I spent on it.
I hate rocket on my pizza!