Northern Ireland is still abiding by EU free movement of goods rules and EU Customs Union rules, and goods from the rest of the UK are inspected when they get to Northern Ireland.

From a trade perspective, Northern Ireland is treated as part of the EU at the moment. This allows the Irish border to remain open, as there's no feasible way of enforcing it otherwise. For some context: The European union as a whole has 137 border crossings with third countries to its east. The Irish-Northern Irish border has 275.

Upvote for politeness :)

Northern Ireland is still saying its goodbyes and looking for its jacket. And even GB is still caught up in hundreds of temporary exemptions and is in many senses operating as part of Europe.