Doug Hamp gives a lucid biblical theology of the New Jerusalem: its nature, history, purpose, size, shape, location, and its timing of when it descends to earth, before or after the millennium. (Incidentally, toward the end of the video he puts a plug in for his book against Calvinism, which I would not agree with, since I believe that God in his unmerited love predestined, before time, a people for himself for his glory and praise, 2 Thess 2:13.) So Hamp gives an excellent outline of what you need to know about the New Jerusalem. Janet Willis fleshes this out in her instructive book What On Earth Is Heaven Like? A Look at God’s City: New Jerusalem.