A topic's article list is ordered by a position set by hand, and then alphabetically by title for everything that has no position. Almost nothing has one, so most of a topic reads alphabetically, with the handful of hand-placed articles gathered at the end rather than at the top.
That is the whole rule. It is not by date, not by popularity, and not by how recently anything was edited. So a new article can appear in the middle of a topic you have already read through.
Previous and Next follow the same list
The controls at the foot of an article step through the topic in exactly the order the topic page shows, and they never leave the topic.
So the first article in a topic has no Previous, and the last has no Next. Nothing is broken and nothing has been taken down: there is simply no article on that side of the one you are reading. To move to a different topic, use the trail at the top of the page or go back to the help home page.
The topics themselves are ordered by hand too
The list on the help home page is not alphabetical either. Competitions comes first and Using UNI comes last, in an order the team sets. Where a topic sits says nothing about how much is in it, and the last topic in the list is the largest one.
Ordering only matters if you are browsing
If you know what you are looking for, use the search box at the top of the help centre rather than scanning a topic. How help centre search matches your words covers what it reads.