We don't know how to build LLMs from scratch either, in that sense. We know how to build a machine that you run trillions of tokens through and hope something good happens. What happens, we only know at a vague and low level. You just end up with a bag of hundreds of billions of floating point numbers with some amazing emergent behaviors.

Similarly, two adult humans know what to do to start the process that makes another human, and we know a few of the very low-level details about what happens, but that is a far cry from knowing how adult humans do what they do.