I wonder what the most complex system is, to date, that has been made and that can all fit in one head. A recent example, though not a program, is Mochizuki's

I wonder what the most complex system is, to date, that has been made and that can all fit in one head. A recent example, though not a program, is Mochizuki's proposed proof of the ABC conjecture. I wonder what program would be comparable to that in complexity.