Originally they didn’t collect much data and used manifests on the individual packages, but for the past 5-6 years they’ve a computer system to enter package in

Originally they didn’t collect much data and used manifests on the individual packages, but for the past 5-6 years they’ve a computer system to enter package info, it’s basically a Google Sheet, and they show the manifest like any other shipping company. They’ll occasionally get a thorough stop but it’s up to chance as anything when at a trusted, high-volume border crossing. If you’ve a Nexus card and nothing to declare, you don’t actually have to stop at the border between the US and Canada if Nexus lanes are open... and if you do have something to declare you can drop off a form. Also they probably stick to the same land border crossings and the same schedule to make it easier long-term. They can close many borders but there a lot of commerce and cross-border travel between Canada and the US and parts of Canada can be more populated/central than parts of the US and vice versa. Besides, as long as the carrier follows directions from CBP or CBSA, the responsibility of the shipment belongs with the shipper and you have to leave your complete ”from” details and they even check IDs, in order to ship with them. They send the list in advance so anything suspicious can be flagged or recorded, that’s how customs works in this digital era...