I really don't understand how complex programs like compilers and browsers have come to an existence.

From little what I know about 8086, it resembles a digital abacus in a way that we can set, push and do all the fancy magic with registers and store them in memories using buttons.

For the like of me, I cannot figure out how an OS was created out of thin air? Do they keep pushing, adding and popping registers back in the day to create OS or compiler? What about the TTY? Was there no such thing as booting? What about file systems? Partitions? How did any of that even work in the first place?