The description in Revelation 21-22 is vivid and specific - measurements, gates, foundations of jewels - which leads some readers to picture an actual, physical city descending one day, and others to read the whole scene as pure symbol for eternal life with God.
One detail is worth knowing either way: the city’s shape - a perfect cube - is the exact shape of the Holy of Holies in the Old Testament temple, the one room only the high priest could enter, once a year. Whatever else it is, the New Jerusalem is described using temple language: it says that the separation which used to keep ordinary people at a careful distance from God’s presence is over, permanently, for everyone who belongs to him. Real place or vivid symbol (or both), the point the text is most insistent on is that access, not exclusion, is the last word.