Flying United to HKG, what to expect?

I need to travel to HKG I often fly One World airlines and know nothing of UA and Star Alliance. One World fares to HKG for my travel dates are nearly double UA's airfare. What are the YYZ - SFO/EWR/ORD flights like? What are the SFO/EWR/ORD flights like? What should I expect onboard UA? I have no status with Star Alliance, jsut looking at the UA flight option as the flight is cheap, money is tight and my travel dates aren't flexible.

Do I need to pay for seats on UA? What are meals like on the long haul flights?

PLease advice.

Thank you