I don't think these people talking about virtual borders and soft borders have ever actually crossed a land border. Frontiers and borders exist whether Brexiters like it or not: they are international facts. You can mitigate them with FTAs etc but you can't just ignore them. It's not Ireland and the EU making this up: the WTO insists on it too. Unless Brexiteers are planning on not even going under WTO rules too....
I went from France to Switzerland this morning. Switzerland currently has a closer agreement with the EU than Brexiteers are planning: they are in Schengen and adhere to the 4 freedoms as part of EFTA. Lots of bilateral agreements with the EU.
I drove through a physical border, guarded by an armed member of the Swiss police. He nodded me through - I'm a middle aged white woman with local French registration plates on my car - but if I had been driving a non local car, an Italian car and/or been a young man of north African origin, the chances are I would have been stopped by this armed man and questioned, and possibly had my car searched.
If my car had Geneva plates and it was a Saturday morning, even a middle aged woman might be stopped and searched to make sure she hadn't bought too much meat or alcohol while in France and that her total purchases didn't go over about €300 and so be eligible to be taxed.
This happens ALL THE TIME on the Swiss / French Schengen border with freedom of movement for people. This is the reality of a frontier and no revamped CTA gets rid of the need for a border, somewhere.
This is what USED to happen crossing the North, but stopped, thanks to a combination of the GFA, which helped remove the armed soldiers and the EU single market in 1993 (which co-incidentally happened around the time of the GFA so people confuse which was responsible for which). Having this in the north is going to be a nightmare. I remember crossing the border into the North many times before 1993. It was not fun. We cannot go back to those days.