Manufacturing of mechanized military hardware (aircraft, tanks, APCs, et cetera) certainly requires both a manufacturing intrastructure and technical experience. It isn’t enough to just acquire an engineering package, go to a mill shop, and say, “Build me this.” A working knowledge of manufacturing processes and how to integrate them into an efficient workflow is critical to producing anything within a reasonable cost and with adequate reliability. There are also the logistics of manufacturing a complex product; no manufacturer is going to be able to produce every component, so they have to be about to outsource the production of some components to other companies so they can focus on their core “competencies”. For a modern aircraft like the F-35, only a small fraction of components are actually built by the final integrator; many major systems such as avionics, engines, fuel systems, weapon systems, et cetera are all built by subcontractors, who themselves have components manufacturered by their subcontractors, et cetera, potentially going down several tiers. Managing all of this requires a great deal of effort to produce a workable product, and even major nations often fail at this (see the history of the Bradley Fighting Vehicle or the Soviet T-72 Main Battle Tank as examples). And of course, all and sundry require access to modern engineering materials such as metal alloys, composite materials, high temperature ceramics, et cetera, many of which are produced specially for aerospace or military applications and are not readily available on the open market.
Small arms are somewhat easier because both the designs and manufacturing methods are generally widely available and the scope of the manufacturing effort can potentially fall within one company, but even these require a knowledge of manufacturing processes and organization which are beyond either the capability or interest of many nations. Still, companies often license or set up manufacturing in other nations and then sell the tooling and facilities to a domestic interest when they are done. This is how Taurus International got started making near-Beretta clones.
An interesting case study is South Africa. Because the country was so isolated during the apartheid era and vigorously anti-Communist at a time that the Soviet Union and China were buddying up to other African nations, South Africa had to develop its own indigenous weapon manufacturing capability. Some of the weapons produced were ingenous; others were interesting but suffered from quality and reliability problems; many were crap. Another example is Israel which is well noted for their internal development of complex weapon systems including what is likely the most effective ABM system fielded to date.
BTW, the reason Soviet weapons (and Chinese copies and adaptations) were so popular among developing nations was that both the Soviet Union and the Peoples Republic of China brought in badly needed hard cash by selling surplus weapons on the cheap. These weapons, like the Kalashnikov series of assault rifles and MiG fighters, were produced in such volume and so cheaply that it was far more inexpensive to buy these weapons than to build them domestically. The Soviets actually outsourced a lot of the manufacturing to other East Bloc nations, so you end up with a SCUD with Romanian markings. This was especially true after the fall of the Soviet Union when everybody was selling all of the surplus and sometimes active weapons to get money to go to the West and buy a Mercedes.
Nations like Syria and Iraq have never made a serious effort to develop a manufacturing infrastructure, and the businesses within that produce manufactured product are usually highly intergrated affairs. On the other hand, Iran has made a very determined effort to develop a completely internal manufacturing infrastructure, and while it is clear that the quality and capability of the products is nowhere near what they would like to advertise, it is certainly improving.
Stranger