I think it's fair to say that we don't have systems anymore that could be 100% understood by a single person. Your smartphone for example, I think it could be 100% understood by a crowd of engineers just big enough to fill a modest concert hall.

Way, way back, I had an 8 bit computer that I built myself from chips and solder. The "permanent" memory was 16kB; I knew most of that code pretty well. Even then the CPU itself required quite a bit of study to comprehend.