Nanobot is only an emerging technology, and its first application will probably be in nanomedicine, including early diagnosis and targeted drug-delivery for cancer, biomedical instrumentation, surgery, pharmacokinetics, monitoring of diabetes, and so on.
The ability for nanobots to build complex devices is far, far in the future, and the way nanobots are shown to work in the Marvel Cinematic Universe films is a practical impossibility, since it apparently involves a low mass of nanobots forming into a very massive suit of armor. It is more magic than technology.