I think “new” are not necessarily topics from the past 2 days. There is a user preference where you can configure which topics appear in new
Similarly, unread only shows topics you are tracking or watching that have new replies (there is a preference for automatically tracking topics after some time of reading; there is also one for tracking/watching on reply, and when creating the topic, you can track/watch all topics with a tag or in a category, and you can manually edit the topic tracking status of each topic).
So I get why it’s complicated. Especially the fact that not all new replies are in unread.
I think it also helped that this was explained on the pages of /new and /unread. Since those aren’t hidden for new users when they are empty, this used to be a great place to learn how these lists work.