Having seen how the insides of ruined buildings look like in Ukraine currently, I wouldn't say it's easier to make a camp in a Ruin... just different.
After all, what's involved in making a camp?
- Set up a sheltered sleeping area: In the woods, that would be sleeping bags and a tarp. In a ruin, that would be clearing off some intact beds or sofas and setting up sleeping bags there, or putting them on the floor in a room that doesn't have cracks or holes in the roof.
- Set up a campfire for cooking: In the woods, that's building a fire with kindling and rocks. In a ruin, that means finding a place where you can light a fire without setting the entire place on fire. Maybe in a bathtub or sink?
- Set up a toilet: In the woods, that's digging a hole somewhat distant from the camp. In a ruin, that means finding an intact toilet or a hole in the ground that leads to lower floors that's still away from camp.
- Set up a lookout: that's pretty much the same for both environments.
For a more permanent stay, I'd say it might actually be harder in ruins, due to the sheer amount of trash you'll have to clear out to make the place livable for a longer period of time.