Heaven, the place where God resides, is in a locale or dimension that we cannot see. The only thing we know about its whereabouts is that because God existed before our time and space, heaven, His home base, is also outside and beyond both. But the time is coming in which heaven will move out of that mystery into the open.
That time has to do with a future era that’s called the New Heaven and the New Earth; so-called because the current heaven will have passed away. (Both ancient cultures and biblical context defines heaven as three different realms – the sky, outer space, and the dwelling place of God. So the heaven referenced in Revelation 21: 1 as passing away, I imagine, is the heavenly sky in which we see clouds and birds.)
The current earth will have passed away too. In the place of the current sky and earth will be a remade sky and a sin-free, remade planet. It is on this new earth, then, that in vision the Apostle John sees the landing of the holy city in all its golden splendor. John observes that the city, the New Jerusalem, comes down from the mystery location where God currently resides (Revelation 21: 2, 10).
We then learn that God, on His throne, has come down with the city. The new earth is now His new address (Revelation 21: 3). The earth we’re on now, but remade, will be the new location of heaven.