There's surprisingly not that much on the net that I'm finding. I learned about it from a traveling exhibit that came to a localish museum to me, but it's all extremely recent (in comparison to most paleontological discoveries) mostly really becoming an overwhelming body of evidence in the last 30 years or so, but the earliest discoveries date to the 70s and 80s.

Looks like PBS Nova did a documentary called "Arctic Dinosaurs" and I linked a BBC article in another comment that gives a good overview as well.

A little bit of reading suggests the arctic during the winter would've gotten down to around -10C so definitely cold enough to require adaptation, but also warm enough to be fully survivable.

Most of these finds evidencing year-round arctic-dwelling dinosaurs is being found in modern day Alaska and Alberta, but there's also similar finds from the other side of the world too!

4-10C warmer on average globally.

The mean arctic circle temp was around 6-7C, so pretty damn warm compared to today.