Well, my first idea is that I should be more careful when typing the word they, since I typed it as "the" every single time.

More helpfully: How much you want to do here depends on how much work you want to put into making this underground world. The other points can be accomplished without actually developing the underground factions. They rely on vague hints and give very little opportunity for meaningful interaction and very little concrete information can be obtained by the PCs. This is intentional since it makes the PCs feel lost, confused, and helpless; things are out of their control. Everyone hates that feeling. When faced with such things, people instinctively retreat to familiar ground. (A cynical definition of "frontier" could be "The place where mankind is still working out local dominance".)

One approach to the last point would be to develop an actual web of subterranean nations, exploring how resources scarcity and lack of light would change their societies and provoke a unique set of cultural values and priorities. This is as much work as creating a new campaign world. Probably more, since you would not be able to draw on as many real world parallels. If you plan on returning, or if you really like the idea, then this might be worth it. Probably not if this is just a few weeks and you don't plan on having it impact the campaign after that.

So that means we need to create the appearance of a complex and foreign culture without, you know, creating one. Thinking "out loud" here:

What would be the main areas where they would be different from us? Food would be a huge one, and land control would be 3 dimensional.

Starting with food: