@abra1de
I’m not an expert, but as I understand it currently - I believe the problem is about collecting tax.
If we left with no deal, the idea is to trade under WTO rules.
That involves collecting tariffs at borders, which would have to involve physical infrastructure (we don’t have tech in place to do it electronically). Hence the hard border.
We could choose not to collect tariffs on goods crossing the Northern Ireland border, but unfortunately, that contravenes WTO rules.
They state that if you reduce tariffs for one trading partner, you have to reduce tariffs for all trading partners. So, it we did that, we’d have to stop collecting tariffs at all for imported goods from anywhere in the world.
The only way out of that, under WTO rules, is if you’re part of a free trade area (but of course, we won’t be).
The hard Brexit “no deal” supporters say there is a technological solution to this problem that wouldn’t require any hard infrastructure, but haven’t ever presented that solution. The upshot is that nobody has any idea if their solution feasible (or even if it exists).
Legally, it seems that we’d have a bunch of contradictory obligations that we’d be unable to square.
As I said, I’m not an expert. I’m happy to be corrected on anything I’ve got wrong.