Fentanyl synthesis without controlled precursors

How do fentanyl synthesis routes differ between powder fentanyl, carfentanil, and other analogs?

Fentanyl and its analogs share a common N-phenethylpiperidinyl-amide core but diverge in synthesis where chemists modify the acyl group at the piperidine 4-position or the phenethyl and piperidine rin...