I've heard in a few cases that people have been detained as they arrive in the airport in Shanghai or Beijing. Agents will escort them off the plane. They are often held in detention for a period of time in the locality where they arrived, the port of entry, and then police officers will arrive from Xinjiang and they will be turned over to them and taken in shackles to Xinjiang, where they'll be processed. It'll be determined in a detention centre whether or not they should be sent to the re-education camp system or if they are guilty of a more serious crime that could result in a prison sentence.
At this point, any Uighur or Kazakh who goes back to China can be subject to immediate arrest. I've heard of a few exceptions, but I've heard many more cases in which immediate arrest has been the result. Even people who had quite significant ties to government authorities in Xinjiang who had tried to ensure their safety when they returned have been arrested as they returned.