“New Jerusalem” is only mentioned in Revelation:
The one who conquers, I will make him a pillar in the temple of my God. Never shall he go out of it, and I will write on him the name of my God, and the name of the city of my God, the new Jerusalem, which comes down from my God out of heaven, and my own new name. (Rev 3:12)
And I saw the holy city, new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband. (Rev 21:2)
It is fairly obvious that it is not a literal city. We are told in Revelation 21:16 that the length, breadth and height of the city are all 12,000 stadia, or 2,200 kilometre. The length of the side of the city, if it were literal, would stretch from Jerusalem to Rome, its area would be more than 100 times the whole area of Israel, and its height would be about 200 times the height at which commercial airliners fly. The angel told John it was a figurative city when he said, “Come, I will show you the Bride, the wife of the Lamb, … and [he] showed me the holy city Jerusalem.” (Revelation 21:9-10) So, New Jerusalem is a symbolic representation of the true church, which comprises followers of Jesus Christ.