"Next to" (or "beside") is just a simple preposition, lenk. The equivalent noun phrase ("the cat next to the box") would just be an myau lenk al piuleng.
A direction word can come directly after the preposition. With lenk "by", it means that the thing is close to the other thing and on the specified side.
A prepositional phrase starting with hu can come directly after a phrase talking about a direction (left or right, also up and down when talking about outer space) to indicate whose perspective this is from.
Hu hyangu specifies that the perspective being used is that of a map, oriented the way maps are usually oriented in this world (west = up).
Hu hyangu is a set phrase, so demonstratives, determiners, and other adjectives are not used with hyangu.