If I am banned from entering the US can I still land at an airport there, for a flight transfer? [duplicate]

No, you cannot.

Unlike most other countries, the US does not allow transit passengers to change between flights without passing through immigration. So everyone arriving at a US airport has to have permission to enter the country.

(There may possibly be exceptions for "tech stops", where the plane lands to refuel but the passengers stay on board. But that's not what you asked about, and anyway tech stops are becoming rare these days.)

When you check in for your flight, the airline will require that you show proof that you are authorized to travel to the US: either US citizenship or residency, a valid visa, or an ESTA if your citizenship makes you eligible for a visa waiver. If you are banned from the country, you will not be able to get any of those, so the airline would not let you board (and might or might not refund the fare for your ticket).

If you did manage to get on the flight anyway, when you landed, you would have to pass through immigration, and they would find you do not have permission to enter the country. You would likely be deported back to your point of departure, or perhaps arrested (you'd have to consult a lawyer on that question).

In the special case of a flight coming from Canada: most major Canadian airports offer "preclearance", where you go through US immigration and customs at a station in your Canadian departure airport. So in this case, even if you somehow made it through check-in, you'd be stopped at the preclearance station before ever leaving Canada.