I think I can raise that to 99.999 % (less than 70,000 people in the 7 billion world’s population) : how a CPU works, at transistor/diodes/resistors level. If the whole Electronics industry disappeared and computers had to be rebuilt using hydraulic valves, I know how to do it. It would take decades and a smartphone CPU will be larger than Manhattan (maybe even more), but I know how to. A more interesting approach would be using some biological or chemical elements that can can perform only two Boolean algebra functions, the NOT function and the AND or the OR function. That would be enough to reconstruct any current CPU.

I’m a retired Electronics Engineer 60 yo, who started fixing computers made on discrete components (Burroughs B500) back in 1975, learning how they work at that level. Later I designed and built my first computer back in 1979, based on Intel’s 8080.