Do I qualify? And how to show proof of "An onward ticket"?

I'm a US citizen traveling from San Francisco, USA to Beijing (flight has 1 stop in Canada), arriving on June 6 at 2:00 pm and leaving on June 9 at 8:00 am to Tokyo, Japan.

Also, I'm booking my trip from SFO to Beijing as a one-way ticket and using a different airline and website to book my plane ticket from Beijing to Tokyo, and back home. How do I show proof of "An onward ticket with confirmed seat"? Do I need to book all the flights through the same website or is it okay to buy them separately?