First, the planet would still be more habitable than space. Second, moon would still be more habitable than space as it has some gravity and plentiful resources. (In comparison to the orbit, which has to import everything except sunlight.) So people would probably live on either the planet or the moon rather than on orbit.
But if you really want to have people living on orbit...
They would get bulk of their food from something like algae farming in tubes using sunlight distributed with optical fibers. Such optical fiber based lighting systems already exist, and they'd work pretty well in the stronger sunlight of space. Without needing large windows or huge amounts of space.
The reason for this is simply that they'd need some extremely robust way of handling the basic carbon, oxygen and nitrogen cycles. You need something that recycles your carbon dioxide and organic waste, and you are not going to want that to be reliant on exports or complex machines.
A system like this would be reasonably compact, fairly low-tech (after construction), and capable of providing basic recycling and nutrients. Food quality would start with tasteless goo, but there is no particular limit on taste given time. Texture could be baked in at the food preparation stage.
That said, they'd probably also have the planet based food sources. I think the scenario was for the evacuation to be fairly sudden and developing robust algae based ecology that supplies all the necessary nutrients would take time. So they'd start with normal agricultural plants and those would never be grown in quantity in space as they are much less space efficient than the algae farms. It would be a long time before the habitats would have enough extra space for normal agriculture.
And lots of these planet based plants would likely remain valued even after the habitats get independent food supply. Imported fruit, vegetables, and herbs would have value as delicacies and justify the cost of maintaining the already built green houses by improving the quality of life in the habitats. And unlike bulk food lifting them to orbit would be a fairly effective way of replenishing the organics lost to leaks and other inevitable inefficiencies of space habitats.
So they'd start with importing all the food from the planet since that is there they have already been producing food and have most resources and infrastructure. Then as the habitats get more self-sustaining and permanent, they'd move to closed cycle production of bulk food in the habitats with specialities and delicacies requiring more space to grow imported from the planet. Given enough time they'd start building gardens on habitats and later expansion of the "green house" agriculture might be built on the moon to take advantage of the free but lower gravity.
All this providing they'd want to live in space habitats...