I was travelling by train from Mongolia/crossed the border by bus because it was cheaper, and when I mean by bus, the bus collects you at the station before the border and drives you to the big border checkpoint building where you cross the border by foot/where they have booths and guards that do the passport and visa checks, just like at an airport.

Maybe I'm misremembering the iris scan. If it did happen, it could be because there were loads of Mongolian students about to embark on their next semester of studies in China.