I think a lot of people underestimate the real power of Evolution. In the fairly recent times, Tibetic, Lepcha, Kirati, Monpa, and some other Sino-Tibetan ethnicities (mix of North Asians and an Ancient Himalayish population), evolved capacity to live in very high altitudes just a few thousand years ago (likely was a rapid evolution). Ashkenazi Jews evolved an IQ averaging well over 12-15 points above the average European (around 99-100), in just a few centuries of taking up intellectual jobs. British Indians are showing rapid evolution in height and IQ, in the last few decades averaging around 97-100+, with increased allele frequencies for intelligence, when their counterparts with similarly good access to nutrition average 84-90 (yes, the same Punjabis, Bengalis, Telugus and Gujaratis who form a large section of the British Indian diaspora, Prime Minister Rishi Sunak being one of them).
These are not random or purposeless changes, unlike what many Evolutionary Biologists put it. I and many other that Evolution is something that is happening deep inside the Quantum and the Relativistic (Spacetime) Realm involving Planck Scale and Sub-Planck scale level of Multidimensional Intelligence, which we will discover in the coming decades (more on this in the chat threads) as we improve our hold and control on Energy Generation and efficiency of use (which is Computation).
Coming back. The Ainu, North Asian and the possible ATL segment of Himalayish migrations into Alaska, Pacific Northwest and, other regions of Canada and Northern USA can easily produce an evolved race fit to build large Civilizations as good as China's, even in Alaska and Canada, eventually expanding into Northern USA, and produce the result that I mentioned.
The only recipe is to generate a situation where there is no other alternative, or just fewer options compared to the OTL. And sustain that situation for a period of 3-4 centuries or a little more. And voila. You have a set of populations as I mentioned, who adapt and evolve to survive in the cold tundra and the steppes of the North America, evolve the strength and intelligence needed to carry forward that Civilization. All this can happen even within 2-3 centuries or a little more, as long as the evolutionary pressure exists. Generating that is the core for this. This isn't a pipedream.