They had been supplying missile technology to the Iranians, the Pakistanis, The Saudis, the Koreans and even the Russians for decades.
They had been building missiles since the 70s, in fact China whole strategy in 90s and early 2000s was asymmetrical warfare against the US and their allies were they will overwhelm the US and their allies air defenses with a sheer number of medium range ballistic missiles, drones, cruise missiles. sound familiar. China production capacity is unmatched. Put in perspective China produce almost a million heavy trucks a year, these trucks are not very different that TEL launchers. These truck can be converted into launchers like North Koreans did when they converted logging trucks into launchers. I think is safe to say that China may have at least dozen of thousands medium to long range variety of missiles and dozen of thousands of cruise missiles. I think China can reach hundred of thousands in a year o two if they decide to do so. Different from other countries, they are almost completely independent in military tech production. Including semiconductors, MEMS, machine tools and so on.