What mundane item is used to replicate a Prosthetic Limb or an Arcane Propulsion Arm?


For the artificer’s “Replicate Magic Item” ability, I’m trying to figure out what mundane item you’d need to start with in order to end up with a Prosthetic Limb or an Arcane Propulsion Arm.

These items seem like very specific magic items. Would starting with scrap metal and gears work? Or would you need to start with a severed limb (flesh and blood)? I’m really struggling to understand the intended component parts of these two particular infusions.