The point is that if the US is a sole producer of a good for which the rest of the world cannot produce a replacement, then it can ban sales of the good to Iran

The point is that if the US is a sole producer of a good for which the rest of the world cannot produce a replacement, then it can ban sales of the good to Iran and also threaten to withhold the good from any nation unless they agree to impose custom controls that prevent export of the good to Iran from their own country. But when the US outsources production to Chinese factories - or when other nations can produce replacement goods on their own - the US can no longer do that, because by definition China can now produce the good as well, and if the US company were to threaten to stop selling the good to, say, Brazil, unless Brazil also promised to not export to Iran, then the Chinese factory would directly sell their version to Brazil instead, and the net result is that the US loses the Brazilian market, but Iran still buys the good. In the new world, there are many suppliers of replacement goods, and so we are not in a position to impose bans on re-export of US made goods to third parties. We can only do that for those goods for which are truly the global sole supplier -- e.g. F-35s and the like. But not for microcontrollers. In other words, the de-industrialization of the West and spread of manufacturing to asia has, as a consequence, severely limited our ability to unilaterally sanction sales of even the goods we produce to Iran - at least for goods that can be resold. We can no longer credibly threaten to sanction any country that resells our stuff to Iran, and thus force them to comply with our sanctions. As a result, even US made products which can be resold are going to end up in Iran if Iran wants to buy them. And this extends even to things like jet engines, so it's certainly going to apply to memory chips, microcontrollers, RAM, and the inconsequential parts "discovered" to be in the cheap Iranian drones. In the new multipolar world, if you want Iran to not be able to buy US goods that can be resold, then you have to actually get the whole world to agree to the sanctions, you can no longer impose the sanctions unilaterally. It is only because people do not recognize this new reality that they are surprised that US made components appear in Iranian drones.