This is where the beauty of the design lies too. The actual diagrams and paper layouts of how it all works and the dimensions of the parts required are pretty much open source material. They are available to anyone who is even remotely interested in rocket tech and does some simple searching. HOWEVER, the workmanship, purity of metals and precision required to build each and every one of the millions of components need to be 100% on point, or they will CATASTROPHICALLY fail during the firing sequence. Every millionth of a millimeter on those miles of tubing that make up the cooling and injection system must be 100% pure and contain an even carbon/alloy content. Otherwise, a single leak during firing due to a weakened microscopic portion on a tube will cause temperatures in that region to instantly reach melting point, which then causes the other regions to melt and leak, and so on, until the entire fuel injection system is destroyed and millions of gallons of volatile fuel and LOX is dumped out of the rocket at once in a flaming cataclysm of destruction. This level of perfection in building these components are unattainable in other countries. It requires absolute skill as well as worker morale and vision/hearing/all other senses of the workers to be at near peak in order to immediately detect and respond to the smallest mistakes that may occur during the manufacturing process. You cannot get this pool of workers in a conscript economy where morale is at the lowest, everybody who is there do not want to be there and are suffering some form of chronic ailments due to a lifetime of near starvation diets, and merit/leadership is based on loyalty to the party and not skill.
It is just like the atomic bomb. A basic gun-type fission device is the crudest of all functional nuclear weapons and the theory behind it is absolutely simple. However, enriching and processing the ore into fission, and then weapons grade fuel requires chemicals and facilities only national effort can produce. That is why we are far more concerned about terrorists getting access to a store of enriched weapons grade fuel, rather than them getting the mere blueprints of a fission device.