144HR VISA: I am a US passport holder and looking to visit Beijing.

Hi, it is visa-free policy, so no visa is needed. Before boarding in US, you need to inform your carrier. Before landing, you need to fill out an Arrival/Departure Card on plane. Then check out and apply for 144h visa-free transit at the immigration counter where you will be given a stay permit with which you are allowed to stay for several days in Beijing.