Read Revelation 21 at Bible Gateway.
Now I saw a new heaven and a new earth, for the first heaven and the first earth had passed away. Also there was no more sea. Rev 21:1
The millennial reign of the Lamb, which takes place in Rev 20, take place in this heavens and in this earth — the one we are living in now. The events of this chapter take place after the millennial reign, after the battle with Gog and Magog, after the beast, the false prophet, and the dragon are thrown into the lake of fire, and death, and Hell, and after the Great White Throne of judgment.
When Satan, death, and Hell pass away, the former things will have passed away, the curse of sin on creation will have passed away. There will have to be a new heavens and new earth, because the laws of physics, that we know, only apply in this universe that is suffering under a curse. The Laws of Thermodynamics, for example, only apply in a universe that is wearing down and wearing out.
And I heard a loud voice from heaven saying, “Behold, the tabernacle of God is with men, and He will dwell with them, and they shall be His people. God Himself will be with them and be their God. And God will wipe away every tear from their eyes; there shall be no more death, nor sorrow, nor crying. There shall be no more pain, for the former things have passed away.” Rev 21:3-4
In a real way, God is dwelling with His children now: His presence inhabits the temples of our bodies by His Holy Spirit who goes with us and who never leaves us or forsakes us. But the Scriptures say that the Holy Spirit is merely a down payment, a guarantee, of what is coming (1 Cor 5:1-5). His presence with us today is not how it will be when the new heavens and new earth arrive.
Then, God will physically dwell with us. We will inhabit the same space and the same plane. We will be together like Adam was together with God in the Garden, only maybe it will be even better than that.
Then I, John, saw the holy city, New Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband. Rev 21:2
In the beginning, God made man, and made them male and female, and instituted the law of the seed and of like kinds. He made these things this way to illustrate His love for us. He made Adam, and gave Him a wife, because He was giving mankind an object lesson in how He loves us. He could have made mankind any way. But He made husbands and wives, because He loves us as a husband loves his wife. He desires to do for us as a husband desires to do for his wife. He desires to dwell with us as a husband dwells with his wife, and cherishes her, and pours out his love to her.
In other words, Rev 21 has been His plan and goal from the beginning. He created man and woman, so that we could experience love as husbands and wives, and then He revealed to us that He desires to be our Husband and He desires us to be His wife. Then we would know from experience, how great His love is for us, and what lengths He would go to for us.
It is the same with parents and children. Bearing children and raising them to adulthood is His idea. He could have designed mankind’s multiplication any way, but He designed it this way. We have an object lesson, of what it means to be a child with a father who loves us. We have an object lesson of what it is like to be a father, and how great of a love that is, that fathers have for their children. The Scriptures teach us that the greatest love in the universe is the love of a father for his children. Then He revealed to us, that He is our Father and we are His children, so that we would know from experience how great His love is for us.
As far as the description of New Jerusalem, this is the dwelling place of the Person and Presence of God. There will be no sun, moon, night, or temple in the city, because God and the Lamb are its light which never stops shining, and its Temple.
The New Jerusalem is built upon the foundation of the twelve apostles, and its twelve gates are the twelve tribes of Israel. This is another way of showing that the two witnesses, Judah and Ephraim, Jews and Christians, Israel and the Church, Old and New Testaments, will be one and not two.
Yeshua is the Root of Israel, even if Jews deny Him. They will only deny Him until God takes the blinders off of their eyes, as He has said. As soon as He removes His hand, and allows them to see, they will see that Yeshua is the Root of Israel. And Christians are grafted in to the olive tree of Israel, even if they believe that the Old Testament has passed away. They will only deny it until God takes the blinders off of their eyes, as He has said. As soon as He removes His hand and allows them to see, they will see that we have been grafted in to the cultivated olive tree of Israel, and are part and parcel of him, expressed through the twelve tribes (Rom 11).
The New Jerusalem is us — collectively, the two witnesses as the body of Messiah — we are the great city, the bride of the Lamb. How can a people be a city and a bride? You know, some things we won’t know until we get there. The Lamb loves us as a bridegroom loves his bride. The Lamb paid for us the greatest price ever that a bridegroom has paid for his bride, because he loved her and went to every length to secure her for his own. The Lamb will dwell with us forever as a bridegroom dwells with his bride, and we won’t know what that is like until we get there. But,
“Eye has not seen, nor ear heard,
Nor have entered into the heart of man
The things which God has prepared for those who love Him.” 1 Cor 2:9