A population of 200 billion would require a significantly different approach than any ideas of bucolic farm fields. Efficiency would be paramount, with soil fertility being likely insufficient.
Cities would have combined facilities for sewage processing and vertical farming.
Nutrients will need to be recycled from sewage. A series of bacterial/algal processing stages would be used to remove pathogens, clean up any toxic contamination and residual pharmaceuticals, and render it suitable as inputs for factory growing operations.
The bulk of foods will likely be derived from mass-production using yeasts and algae creating a nutrient paste. Endless vats stacked together with lighting and aeration and fed through pipes in a continuous flow - sewage in one end, nutrient paste out the other (obviously there would be some loss with sludge from unusable materials/contamination too - no system is perfectly efficient). The wealthier you are, the less of this you will likely eat, but most people will have at least some of this to pad out their diet (usually isn't meant to be eaten by itself but as just one ingredient in a dish).
Vertical farms growing higher-value produce would provide the taste and mouth-feel and simply better satisfaction of the meals. Stacked racks of plants grown under artificial lighting, fed on a nutrient flow (coming from post-treatment sewage/compost), and being the advanced form developed from the modern vertical farms you can see being developed today. These will likely be robotic facilities, so full of CO2 for faster growth (doesn't need to be breathable for humans, and lower nutrient density isn't important since those come from the nutrient paste).
They likely also have annexes for turning some of their waste into insects - something like black soldier fly larva or mealworms as protein supplements, and to feed some fish (aquaponics) or chickens for eggs. If there is a good system for collecting food waste (which there really should be), it could be used for more expensive foods like pigs raised on table/kitchen slops.
So far all of this is expandable up to any scale necessary, but some land will still be used for traditional farming. This will be a combination of having expensive "natural" foods for the affluent (not affordable to the common population beyond special occasions) and because grazing animals are great for land conservation. This makes dairy more of a special luxury product, and meat from livestock a rare delicacy, but they are unlikely to disappear entirely.