A person, which lives and works in USA, has dual citizenship Italian and US, what will the foreign national potential spouse (from China) have to go through in order to be legally residing in the US? Should they get married in US to simplify the process? Can they get married in Italy?
Most countries recognize marriages performed in other countries, and the US is no exception. The couple can marry in any jurisdiction that will marry them.
The advantage of marrying in the US is that they will not need to get the marriage license legitimized before presenting it with the application to the US federal government. This may be a relatively small benefit.
The Italian nationality of one party may affect their ability to marry in Italy under Italian law (I do not know the requirements to get married in Italy), but it will not have any impact on recognition of the marriage in the US.
The specific steps to legal residency depend on whether the foreign fiancee is already in the US (and, if so, in what status), and on whether the couple decide to marry in the US or elsewhere.