Of course many of UA's Pacific routes (and early planes, B747SPs) came from PanAm and originated as flying boat routes after WW2...some even pre-War. HKG and Canton were the original China ports of entry, and after Mao's victory the west was closed out of all ports of entry, leaving HKG and Taipei. Civil aviation was not developed enough pre-WW2 to have either Shanghai or Peking as major flight centers though the former did have short haul regional links growing with HKG and TPE. Of course, Japan was an enemy at the time and had occupied the Korean peninsula and northeastern China. And of course airplanes had much shorter ranges and catered mainly to diplomats, millionaire travelers, and a few hardy businessmen (gender intended).