https://en.m.wikipedia.org/wiki/Good_Friday_Agreement
Just finished reading this wiki page. Does it accurately capture the issue?
As I understand it the EU will have the authority to create and enforce a border between NI under UK rule and mainland Ireland which will remain EU post Brexit.
Bottom line. Just because the EU will have authority to enforce a border they are under no obligation to do so and that should be if no concern to UK post Brexit. Correct? Or am I missing something?
I would suggest reading the text of the agreement itself.
The rest of your post is inaccurate. Post Brexit there will be no border on the island of Ireland, just as today.
But that goes against the Brexit “take back control of our borders” so as a compromise, the customs border will be in the Irish sea between northern Ireland and rUK.