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The Millennial kingdom is on earth, with biological inhabitants, and incorruptible inhabitants who have returned to the earth with Yeshua, ruling over them.
The “New Jerusalem” is after the earth and heavens have “departed from his face,” and is populated by the bride in their incorruptible bodies.
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Actually, now that you mention that, you are correct. The “millenial kingdom” is only talked about for a couple verses in chapter 20 before the final battle with Satan and the last judgement, and the New Jerusalem is described in Chapter 21, after the last judgement.
What is interesting though, is that Chapter 21 begins with:
“And I saw a new heaven and a new earth: for the first heaven and the first earth were passed away; and there was no more sea.”
Now, MHGinTN claimed Isaiah 65:20 is a description of the “millenial kingdom”, but Isaiah 65:17 says:
“For, behold, I create new heavens and a new earth: and the former shall not be remembered, nor come into mind.”
This matches the introductory verse to Revelation 21, so if the events of Revelation 21 take place after the “millenial kingdom”, then the events of Isaiah 65:20 must also take place after that time.