1. The New Jerusalem in the eternal home of the saints (Heb 12:22-24). It is spoken of in the present tense here since God already sees all His saints in this city, and Abraham himself had a vision of this city (Heb 11:10)
2. The New Jerusalem is presently located in Heaven (with Paradise within), and it is a cubic city which is the size of a continent -- 1500 cubic miles (Rev 21:16). It is cubic because the Holy of Holies was a cube, and this is the ultimate Holy of Holies.
3. The New Heavens and the New Earth are established before this city descends from Heaven (Rev 21:1).
4. The New Jerusalem descends from Heaven but hovers over the earth and gives light to the earth (Rev 21:2,11,23,24)
So rather than being uninhabited, the New Jerusalem is inhabited eternally by (1) God, (2) the Lamb (Christ), and (3) the Lamb's Wife (the Church).