Visa exemption. I am an Australian citizen travelling from Melbourne to Beijing

in mid-October. I am planning on staying in Beijing for three nights, and then I am travelling onto Canada.
1. Am I eligible for the 72 hour or 144 hour free transit exemption?
2. When does the 72 or 144 hours "start"? I have read conflicting information around whether it starts the time your plane lands, or at midnight the day you arrive. For example, if my flight arrives in Beijing at 7am on 13th of October, does that mean I must leave by 7am on the 16th of October, or by midnight on the 16th of October? This affects the time I book my flight to Canada, so I was hoping to get some clarity on this!
3. Are there any requirements that I need to know about? For example, is there anything that I need to do or prepare beforehand? Do I need to fill out an application, or is the visa exemption arranged and processed on arrival in Beijing International Airport?