Nano-tech future from sci Fi ran into thermodynamics. Heat disapation for the work to be done is a pain to disperse when doing nano-assembler work.

On the other hand, spread it out over time, and we've had it for millions of years.

Look at an acorn. Self contained, self replicating instruction set. Just add nutrient solution raw material and energy, and it builds a huge tree, in decades.

Total vonNeumann machine.