Smuggling within the common market/customs union is an issue of revenue loss and it is down to the Irish/N.I authorities to manage that, which the decision has been to use intelligence to try to crack down on the most serious offenders.

But once N.I leaves the common market/customs union, that is a whole different ball game. Any products say food or medicines has to conform to EU standards. Anything produced in the EU can be inspected by EU verified inspectors so it is allowed to travel freely around the EU.

Now the UK wants to "take back control", so the EU has to take it on trust when the UK says their products meet EU standards. As you say they will sort that out so company X will be able to export product Y into the EU. At the ports they can clear that product to allow it to pass through with little or no delay.

Now how do you do that with a random truck driving across the N.I land border?
Now this is an EU matter, as the integerity of the Single Market comes into play, and the tolorance of a certain amount of illegeal activity is no longer acceptable