Service on a defendant in mainland China has one dependable route: a request under the Hague Service Convention to China's Central Authority, the Bureau of International Judicial Assistance at the Ministry of Justice in Beijing. China has objected to every alternative channel in Article 10, so documents can't be served by mail, and a private agent handing papers to a defendant in China doesn't produce valid Convention service.
Once the Ministry of Justice accepts a request, it passes it through the Supreme People's Court to the local court where the defendant is located, and that court carries out service and completes the certificate. The process is thorough but slow, and a request with an incomplete translation or a missing signature can sit without any notice to the sender. Getting the package right before it leaves the United States is the single biggest factor in how long service in China takes.