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First, Canada does not have a general policy allowing visa-free transit (even if you remain in the international transit area). If your nationality and itinerary require a transit visa pre-Covid, you need a visa under Covid rules as well.
travelers from the US (and only the US?)
The transit exception applies to all foreign nationals departing from countries other than the US, as long as they remain in the sterile transit area and are travelling for a non-discretionary purpose.
The exemptions under the Orders do not apply to foreign nationals travelling for optional or discretionary purposes. The onus is on the foreign national to demonstrate the purpose of their travel.
The onus is on the foreign national to establish that the travel is non-discretionary (e.g. work, study, assisting a family member).
For persons travelling from the US, as long as the purpose of travel is non-discretionary, they may enter Canada for transit purposes.
Examples of other non-optional or non-discretionary travel: [...]
transiting through Canada for non-optional or non-discretionary purposes
There is no requirement to stay within the sterile area if you come from the US, but if you leave the area, you become subject to possible public health measures.
If you cannot demonstrate that you are travelling for a non-discretionary purpose, you may be denied entry/transit regardless if you come from the US or not.
are allowed to transit without completing any additional testing or checklists
For passengers staying within the sterile area, ArriveCAN is required for transit passengers, but pre-arrival test may be exempted. You are also exempted from arrival testing, hotel stopover and quarantine if you remain in the transit area. (See rubric "Transit" at https://travel.gc.ca/travel-covid/travel-restrictions/exemptions).
If you intend to leave the transit area (or must do so because of your flight itinerary), then you may be required to be tested and subject to quarantine until your next flight. So you should inquire with the airlines and the airport for the exact itinerary.