The UK is an Island nation, and so has never has had a land border with the EU . That is why we have had a special unique place , politically inside the EU yet physically outside.
But Brexit has ensured that we now WILL have a land border with the EU. The one between Northern and Southern Ireland. Both the Northern and Southern Ireland governments insist that that border can never be hard, closed, controlled or restricted in any way what so ever.
Scotland and London have both said that if Northern Ireland has any type of special relationship with the EU as in the form of any type of border, then legally they MUST also be allowed a border with the rest of the UK. Which means they must be allowed to stay in the EU and the UK with Scotland.
(as NI is just as much part of the UK as they are.)
As far as I am aware no one has yet come up with a solution to this problem, they are just hoping it will go away. Possibly someone knows the answer? if so please tell the government.