https://inews.co.uk/news/politics/brexit/brexit-deal-done-northern-ireland-customs-consent-explained-816194
'Customs
Article 4 of the Northern Ireland Protocol clearly states that: “Northern Ireland is part of the customs territory of the United Kingdom.”
Practically speaking, this means Northern Ireland will be included as part of, and benefit from, all future trade deals the UK makes with third countries after Brexit.
However, to get around the problem of a hard border, including checkpoints, with Ireland, Northern Ireland will still come under some EU customs rules - meaning any goods that come from Britain into Northern Ireland will be subject to an EU customs code.'
That shows Ulster will be treated differently from Great Britain.