(1) The first heaven is the air around us, the blue sky and clouds above us (the atmosphere), and the vast black night sky celestial dome of outer space(spacetime), all created by the eternal uncreated Creator, God/Jesus. In the Bible see, Genesis 1:1,2. This is the space of the sun and moon, planets, stars, black holes, and galaxies. In the Bible, this all collectively is the firmament(the sky).
(2) the second heaven is beyond. It is not necessarily a timeless heaven. It is where the angels dwell, who have access to our universe(the first heaven) and the timeless higher(highest) third heaven; though, this intermediate second heaven is Bible interpretation. The higher(highest) third heaven is where God the Father is. It is the heaven revealed by God/Jesus only to Paul. It is the heaven of our heavenly hope, where Jesus is preparing a place for us, after this life; otherwise, you could split the first heaven into two heavens—-the atmosphere, and outer space, where our atmosphere would be the first heaven and outer space would be the second heaven. The Bible doesn’t specifically tell us; rather, it is Bible study interpretation—-“For now we see in a mirror, dimly, but then face to face. Now I know in part, but then I shall know just as I also am known.” I Corinthians 13:12—regardless of how you define it, the first and second heavens are the realm run and rule of the angels—-maybe Lucifer/Satan(the Devil) the fallen highest angel, and his angels (the fallen angels that followed Lucifer when God cast him out of or denied him access to the third heaven, are what aliens are. That is just a passing speculation about aliens to consider, but for more on that see, post(chapter) 20. My Idea on Extra Terrestrial Life.
(3) the third heaven, which is something timeless beyond that, is the eternal heaven where God dwells. It also is where Jesus is preparing a place for Christians to live forever and ever with Him, eternally, higher than the angels, when He takes us there after having our raised/transformed new body, at the resurrection/rapture, if we believe in Him. There could possibly be some sign of a second heaven’s existence (but we will probably never be able to know about it in our world because the accelerated expansion of our universe has put it beyond our horizon to know about it today.—-“‘Let not your heart be troubled; you believe in God, believe also in Me. In My Father’s house are many mansions; if it were not so, I would have told you. I go to prepare a place for you. And if I go and prepare a place for you, I will come again and receive you to Myself; that where I am, there you may be also. And where I go you know, and the way you know.’” John 14:1-4—“For our citizenship is in heaven, from which we also eagerly wait for the Savior, the Lord Jesus Christ, who will transform our lowly body that it may be conformed to His glorious body, according to the working by which He is able even to subdue all things to Himself.” Philippians 3:20-21—the third heaven is something beyond an angelic second heaven and beyond our first heaven universe of planets, stars, and galaxies, this is the third heaven reveled only to Paul by God/Jesus when the bright light shone down on Paul and he fell to the ground on the road to Damascus.—-“I know a man in Christ who fourteen years ago—whether in the body I do not know, or whether out of the body I do not know, God knows And I know such a man—whether in the body or out of the body I do not know, God knows how he was caught up into Paradise and heard inexpressible words, which it is not lawful for a man to utter.—such a one was caught up to the third heaven.” II Corinthians 12:2-4—The third heaven can scientifically be thought of as a somehow higher parallel universe; though, the third heaven is an eternal uncreated timeless highest heaven that always existed. It is where the uncreated God dwells eternally, and is where Jesus is now, seated at the right hand of the Father, and preparing a place for us(Christians) to live with Him eternally.
It is interesting to try to imagine how timelessly wondrous the third heaven must be. It must be as wondrously far beyond the the first and second heavens, as outer space is beyond the atmosphere. T Rh he third heaven is not one of the Hugh Everett many-worlds universes in the multiverse, though the second heaven might be; rather, the third heaven is something timeless revealed by God/Jesus only to Paul, as Paul writes above, it is something he could not put into words. That is also what Paul means when he says, “not lawful,” the words would be too grandiose. It is where we/Christians will be with Jesus forever and ever eternally.
No one can prove the the second or third heavens exist, but no one can prove they don’t either. It’s taken on free will faith and Bible interpretation as to what are the first, second, and third heavens. There is nothing to lose by believing and everything to gain(no fear of death and the gift of eternal life in heaven), and everything to lose(resurrection judgement, and eternal death(know-nothing sleep) by not believing. It’s very simple. The big things are.
It is a great question often asked: where did our universe(the first heaven) come from? The true Biblical answer is: it was created by God, by way of the timeless uncreated third heaven. This would be where the, I AM, the Genesis Force came/comes from—the Fifth(First) Force, sprang and will return.
(See post(chapter), 40. I AM, the Genesis Force—the Fifth(First) Force).