A lot of times, "knockoffs" aren't even that. If you take a knock-off to be a close approximation in appearance to a known brand, in some cases you'd be wrong. In some cases they don't even have to design or reverse engineer the product. "Second Shifting" is a thing too. A chinese contractor that has an order for 20,000 widgets already has the tooling setup and the materials at hand. It's no problem for the night shift to keep the machines running for their own private production run of an identical product (but often using cheaper then specified materials) and just sell the result on the gray market. No approximation or development needed.