What is considered port of entry?

If you fly from Kathmandu, Nepal to Beijing, but have a change of planes in Guangzhou (1.5 hr in transit) and then disembark in Beijing (final destination in China), and then depart Beijing 3 days later to U.S. again changing planes in Guanzhou (2 hrs in transit), with final destination the U.S., is Beijing the port of entry for China, and does this routing qualify for the 144 hour visa free transit? The stops in Guanzhou are only to change planes, not leaving the airport.