Do I need a transit visa? If yes, where do I need to apply for it?

I am an Iranian citizen. My ticket Tehran to Seoul has 2 stops in China: First in Urumqi for less than 3 hours, second 4 hours in Beijing. I don't leave the airport during my stops. In addition, my plane lands in Urumqi and leaves China from Beijing within 12 hours . I am working in Korea and have a valid ARC card. Do I need a transit visa? If yes, where do I need to apply for it? Thank you.