They are most likely using resolution via the Pope–Peachey Method. There was supporting evidence found in a laboratory in Guadalajara[0]. It is possible to do

They are most likely using resolution via the Pope–Peachey Method. There was supporting evidence found in a laboratory in Guadalajara[0]. It is possible to do asymmetric reductive amination using enzymes but this is out of the scope of clandestine producers. Likewise a chemical and not enzymatic asymmetric reductive amination would be easy in a bench lab, probably to expensive and impractical in a clandestine setting. [0] Joseph S. Bozenko, "Clandestine Enantiomeric Enrichment of d–Methamphetamine via Tartaric Acid Resolution", JCLIC, 2008, vol.3 (not publicly accessible but you can find this if you know where to look)