system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do countries that are at war with each other interact in the UN?

 Do they still attend the same meetings? Do they cut all ties?
gpt
So actually declaring war and breaking off diplomatic relations are two separate processes. You can cut off diplomatic relations with someone without declaring war on them and you can declare war on someone without cutting off diplomatic relations. In actual fact usually you cut off diplomatic relations with someone in order to warn them, and avoid a war; once war is joined that strategy clearly hasn't worked and so you actually start to rebuild diplo relations, because you need them to negotiate peace etc..

Often you don't want to lose face by reestablishing relations in public with your enemy and so the first diplomatic contacts will be secret. So for example Kissinger's secret first talks with North Vietnam.

The UN is different though. One of the great values of the UN is that even when countries aren't willing to have relations with each other, they both have relations with the UN. And so the UN becomes a mechanism for enemies to talk. Mostly indirectly, but occasionally ambassadors will chat in the corridors, or address each other in/through GA or committee meetings.

Edit: you might hear people talk about "track one" and "track two" diplomacy. Track one diplomacy is official discussions between nations. When those nations are at war track one isn't really possible except by formal peace talks. Track two is informal discussions, or discussions by non-state actors (NGOs academics etc..) on the state's behalf. Most peace processes start as track two initiatives.