The UK and Ireland impose passport controls on the EU (Common Travel Area) which won't change, with regards to customs checks we're engaging in a bit of legal t

The UK and Ireland impose passport controls on the EU (Common Travel Area) which won't change, with regards to customs checks we're engaging in a bit of legal theatre to claim that NI is remaining inside the UK customs area but de facto it will be in a customs union with the EU/the Republic of Ireland and the customs border will be between Great Britain and the island of Ireland as a whole.