TL;DR Gallium and Germanium. These are mostly used in novel III/V semiconductors, rather than mass produced silicon chips.
China doesn’t have the only source of these metals. They are everywhere. However, processing them is incredibly toxic and dangerous, making the process very expensive. Unless you are China and don’t care about the environment. So basically this cuts us off from cheap sources. Just like cutting companies off from cheap labor means no one wants to work, companies being cut off from cheap metal means there isn’t an alternative.
Yay, we restrict their access to silicon etching machines, and the return with metal restrictions.
2 wrongs surely makes us right
Why are those wrongs? They are both first steps to technological self sufficiency. Which are both good things from their individual perspectives.
Every time China tries this other countries immediately start up their own production and China eventually folds and allows exports again so they can keep the export market. They have nothing here.