Technology

GPUs that are banned from export to China can be easily acquired in low numbers in the country due to individuals bringing in banned GPUs which get sold just like unbanned GPUs after passing through several people. Also possible that some banned cards are repaired QC failures from the Chinese factories that produce these cards (the cards are produced in China, but are supposed to be exported and banned from export back into China). The export restrictions present more of a barrier for building out large AI clusters with many, many GPUs.
Nvidia almost certainly knows about this but turns a blind eye because they want to sell more cards and China is a large market.
There are some repair shops in China that can not only repair the GPUs, but can also transfer the GPU die onto an aftermarket PCB and give it double the amount of VRAM to create a card that Nvidia doesn't even sell.
Individual banned GPUs are easier to get than the full systems that Nvidia also sells. AMD GPUs are much less popular, Intel GPUs are basically non-existent. The overall most popular card for smuggling into China seems to be the RTX 4090.