How is China cut off from global market? Even Huawei smartphone still can be shipped to US thru parallel importer.Cut off from the global market & access to critical technology is pretty much a death sentence for any company.
If China didn't have such a large population & state intervention Huawei would have gone under years ago.
If the world is cut off from China, apple and many US tech firm will go bust. Currently , all apple phone is made in China.
You seems to have mix up between cut off from US tech supplier for parts vs cut off from global market...