https://www.canada.ca/en/immigratio...ovid19/visitors-foreign-workers-students.html
Some people don't realise, or forget, that we live in a global society. Every day there are people with dual (or more) citizenship, permanent residents, essential workers, low wage foreign workers, returning travellers who only now are coming 'home', and international students travelling globally,
including to and from Canada. The China-Canada flights have been very busy transporting international students for instance, along with people moving around for other purposes. Remember that Trudeau never stopped those flights earlier in the year. Some carriers never stopped Canada-US flights during the entire period, and are having to increase capacity. Aeroplan members over on Flyertalk are booking Canada-US flights at the new promo like crazy, even under the current restrictions.
I do think that Canada will open to the EU before they open to the US.
As to your question, there are people flying China>Canada>US for example every week. If they are US citizens, they can of course not be banned from entering the US. The airline does check but there is a lot of discussion of when (ie first point of departure? transfer point?) and how well they are applying any limits. Timatic is not always updated as the restrictions have changed so much, so that adds an extra element. From what I have heard, the bigger challenges are people trying to fly via some Asian countries ie Singapore kept changing who could transit so someone flying Australia>Asia transit?>Europe could not always find a way home.
My understanding is that in the example above, there are more travellers from the US who are denied entry by CBSA and returned to the US than from Asia, Europe, etc.