The New Jerusalem is a physical reconstruction, spiritual restoration, and a divine recreation of the city of Jerusalem. The Bible states that it will also be the dwelling place of the saints. Outside of the City will be the remainder of the New Earth, while the city will have gates they will never need to be closed. All believers living in the New Jerusalem, will have access the New Earth for enjoyment.
There will not be ‘nations' in the Eternal state, simply the Jewish-Gentile distinction (without functional difference) will remain throughout eternity.