To paraphrase Dr. Ralph Merkle of Xerox PARC, everything is made of atoms. If you rearrange the atoms of a lump of coal, you make a diamond. Rearrange atoms of sand and make computer chips.
Potatoes rearrange atoms of soil, air, and water to make copies of themselves.
If we can build objects on such a fine scale, we can make extraordinary things from ordinary matter. The key to self-replicating technology, natural or otherwise, is premanufactured, perfectly identical building blocks. This means atoms. Also essential to self-replicating systems is the ability to move and manipulate individual and groups of atoms with resolution at least as good as or better than an atomic diameter. No easy task.