The hellenistic culture is the greek culture that is characterized by post-Alexandrian influences. This means that when we're talking about the hellenistic culture, we're talking about Greek culture which has been influenced by Alexander the Great and his conquests around the world. The main difference between the two in all aspects was that the hellenistic culture was much more diverse in all regions of life as foreign people, not originally from Greece influenced daily life as well.
The southern slopes of the Himalaya receive more rain due to the south west monsoon winds which travel west along the southern slops. The northern slopes do not receive any such rainfall. So the southern slopes are covered with thick vegetation as compared to the northern slopes.
Steamboats connected the north and south by opening up new trade and travel routes along the rivers. Additionally, these are all relatively large cities so I would presume they were centers of commerce.