“What if anything does the Church officially say about the New Heaven and New Earth in Revelation Chapter 21?”
She says a lot of things in the ordinary magisterium, which basically means you’ve got to look up all kinds of old theology and Biblical commentary books to find out.

There have also been some papal documents about heresies and errors dealing with the life to come, so you can use them.
A good place to start, however, is the Catechism of the Catholic Church. Here’s the section on
“Hope of the New Heaven and the New Earth”. There’s also Article 1060 in the “In Brief” section right afterward, which notes that the material universe will be transformed.
“Gaudium et Spes” has this lovely section recapping Church teaching and Scripture:
“We do not know the time for the consummation of the earth and of humanity,(15) nor do we know how all things will be transformed. As deformed by sin, the shape of this world will pass away; but we are taught that God is preparing a new dwelling place and a new earth where justice will abide, and whose blessedness will answer and surpass all the longings for peace which spring up in the human heart. Then, with death overcome, the sons of God will be raised up in Christ, and what was sown in weakness and corruption will be invested with incorruptibility. Enduring with charity and its fruits, all that creation which God made on man’s account will be unchained from the bondage of vanity.”
“I think I’ve heard that Heaven and Earth in Revelation 21 will no longer be separate but will be one.”
Well, obviously we don’t know for sure, but the nuptial symbolism of the Bride somehow also being the New Jerusalem coming down from Heaven, and of God dwelling among His people in the New Jerusalem to such an extent that He’s their light to see by, etc., etc. seems to say that Heaven and Earth will at least be joined together/wedded, if not actually smooshed into one.
Bede, for example, notes Daniel’s vision of a stone not cut by hands (ie, Jesus) turning into a mountain and filling the whole world. He sees this as a prophecy of the time of the new heavens and new earth - that Jesus will visibly fill all things in some way, as well as the City.
There are all kinds of theories, many very lyrical. However, pretty much every teaching on this subject acknowledges that we don’t know how the prophecies will be fulfilled; and that what God has coming is beyond our imagination. So it’s pretty much all educated guessing and not much formal teaching from the Church. God is telling us to wait and see.
You can look for words like “consummation,” “renovation,” and so on.