They definitely are using Nvidia. Part of deepseek's special sauce was using an "undocumented" ptx instruction to get a cute microoptimization with the memory hierarchy.

https://youtube.com/watch?v=iEda8_Mvvo4

They don't work. Chinese are skilled enough to desolder and smuggle just the ships themselves. They make the rest of GPU in-house. With more VRAM than the nvidia offers, comically, in case of 4090.