There's a level of food sufficiency one can achieve on their own, even without land. e.g. I'm putting together a medium scale hydroponics system right now for home - it needs 100-120cm of wall, 30cm deep at its thickest point, and can grow about 120 separate spots.

Now obviously hydroponics is not an easy answer for all food needs. You can't easily grow wheat and other grains/cereals in the volume you'd need. You can't grow larger fruits that require trees. Smaller fruits - tomatoes, cucumbers, berries are usually pretty doable. And of course tons of herbs and spices - rocket/arugula, basil, mint, you name it.

If anything, the herbs alone are worth setting up a small home unit, especially since it's so effort-free - add water, measure nutrients, repeat once a week, harvest in 3-4 weeks earliest, trim it proper and freeze for long term storage, and you don't even need to re-plant things because most of what you'd grow in hydroponics, when trimmed right, just grows again.

Again, this doesn't solve all the issues, but having some freshly grown herbs at least is a good way to get started with "grow your own", even if on full scale it's impossible.