So, it's the future and society has long since became divided upon distinct lines of rich and poor. The rich upper-class living a life of luxury and the poor lower-classes living each day at a time, thousands of meters above the Earth in the sky, in massive balloons and the tops of skyscrapers, the ground reserved for the rich, but why?
What are the poor doing up there, what work is being done up there, where the air is thin?
Menial jobs are there everywhere - even up there
There are plenty of menial jobs that may need to be done in the sky. Here is a selection:
Courier - People still order stuff, and especially if they are wealthy they don't necessarily want to go get it. So convenience often trumps effort, and even today with the emergence of instant ordering with next-day delivery this has spawned a whole new couriering industry.
Garbage Collection - Currently we have many trucks going around cities to both poor and affluent alike, however perhaps in your world garbage collection is too slow on the ground, or in fact the wealthy don't like 'seeing' them at all. And if collection occurs in the air, perhaps it's convenient to sort out recycling and process it up there too.
Servicing - the wealthy have machines that break down, jewellery that requires resizing, fabric to be repaired. It is conceivable in the ultra-convenience future that the poor who do these menial tasks live a nomadic life and need to be ultra-accessible.
Communication - if it is a high-tech world, perhaps equipment to communicate for the wealthy. We currently have towers and antennae to do this, but if they for some reason are not desirable, then it makes sense that these are located in the air. They need maintenance. If a low-tech world, flag signals and a network of postal / visual communication requires a whole echelon of people up there.
Entertainment - contrary to belief, entertainers are often poor, living lives with no central base. Circus performers, theatre actors are all in this class, and easily could be up there. Prostitution perhaps could be in this category too.
In fact, think of what the poor in cities do now. They are often transient, unable to retain property, come from disadvantaged backgrounds so are unable to break the cycle of poverty. Their work tends to be menial as they do not usually have the education and connections commonly entitled to the wealthy middle-class. They generally then do menial labour, finding work where they can. A nomadic life in the air is actually quite likely if they can't break the cycle to own property on the ground, living off the scraps and outcomes of the more established.
Jobs wouldn't differ that much from those on Earth. Manufacturing would still be common (though arguably mostly automated), and many goods and services will still need to be produced.
The one notable exception I can think of is agriculture. Without land for crops below, working people will have to grow crops up above. Luckily, aeroponics (and hydroponics) are far more efficient than traditional farming, so they make sense given spatial constraints.
Mining
Someone needs to harvest and refine all the minerals used to keep people up in the air in the first place. I imagine some surface land will be used as the entrances to deep mineshafts, in which some lower class citizens may work. This could evolve into a feudal system of wealthy landowners leasing land.
If you really want to keep them up in the air, though, asteroid mining is the next best thing. The problem is, the required rockets are too expensive for sky dwellers, unless they're owned and operated by the rich.
