It's smuggling, stupid
Even if we believed the typically delusional Brexiter thinking about miraculous, invisible technical solutions to not only tracking but verifying the content of purportedly legitimate traffic across the border, this does nothing to deal with the 900lb gorilla in the room: smuggling.
The whole point of Brexit—insofar as there is any point at all—is that the price and/or desirability of some goods will differ on either side of the border. If they didn't, NI would still be in the EU. That difference is the motivator for smuggling, which can be hugely profitable. I am here to tell you that unless certain experiences in the 1980s badly misled me, NI is as well-stocked with folks with the aptitude and inclination for criminal enterprises as anywhere else on earth.
In short, as anyone with a scrap of common sense can foresee, smuggling will assume an epic scale. Throughout the whole of human history, the only way to prevent it is to inspect goods crossing the border. And you need to do it at the border, not 50km away after all the goodies, and baddies, have been methodically dispersed.
Suffice to say, the inner Ireland border (ha: echoes of the old IGB) is unfathomably complicated. Supposing, though, some invisible network of tens of thousands of cameras and motion detectors were able to survey every car, milk truck and delivery person crossing the border about their business 20 times in a day: whom would you target? Even large-scale smuggling would be a drop in the ocean of legitimate movements—and you can be sure that the smarter crooks will go a long way to disguise their operations.
There cannot be border infrastructure.
Illusory technical magic cannot replace border checks.
Large-scale smuggling would be both inevitable and unacceptable.
A border in the Irish Sea is hilariously unworkable.
Therefore, NI has to remain within the EU's customs union and single market.
Therefore, Brexit can't happen.
Those pesky facts won't go away. No matter what hysterical lies and fantasies emerge from No 10.