Hardly. The US is still a large trade partner for China, and an immediate stop will certainly trigger a domestic crisis, but they aren't really that dependent on the US. I'm not aware of a single item that only the US exclusively produces.

OTH China invests 95% of GDP on infrastructure (compared to 1.5% in the US), creating a huge domestic market. And a a stop of trade with the US will also mean a stop of China keeping the dollar strong - which they do because their currency is pegged to the dollar. They won't have a reason to continue this practice without trade with the US.

Iran is pretty far away from the stone age, despite US sanctions. If the components can't be sold legally, they can still be smuggled. The added inefficiency would of course drive prices up, but it wouldn't be quite as disastrous as you seem to believe.