We're introduced to the concept of fractal neuronic cloning by Agnes in episode one, who says it was what Maddox was working on before the ban. Soji and Dahj are the product of that process, created by Maddox. Soji and Dahj are created from Data's neuron. Therefore Maddox had Data's neuron and took it with him to Coppelius. The rest should fall into place pretty easily.