The short answer is that prior to the end of the Jewish age people died and slept in Sheol/Hades, but since the Day of the LORD, things are different.
The longer answer is:
The “key” to understanding the death experience in scripture is, I believe, found in 1 Corinthians 15. Unfortunately, most people understand this passage as referring to resurrection:
[1Co 15:51-57 NLT] (51) But let me reveal to you a wonderful secret. We will not all die, but we will all be transformed! (52) It will happen in a moment, in the blink of an eye, when the last trumpet is blown. For when the trumpet sounds, those who have died will be raised to live forever. And we who are living will also be transformed. (53) For our dying bodies must be transformed into bodies that will never die; our mortal bodies must be transformed into immortal bodies. (54) Then, when our dying bodies have been transformed into bodies that will never die, this Scripture will be fulfilled: "Death is swallowed up in victory. (55) O death, where is your victory? O death, where is your sting?" (56) For sin is the sting that results in death, and the law gives sin its power. (57) But thank God! He gives us victory over sin and death through our Lord Jesus Christ.
In reality, (in my humbler than thou opinion), he is revealing a “wonderful secret”… that death is not going to be the experience of everyone. In the past, everyone died. Not so in the future.
Now, the first century Christians, when they spoke of “death”, they knew enough from Paul’s writings that it wasn’t really death, as a permanent fate, but only a nap; “sleep”:
[Jhn 11:11-14 NLT] (11) Then he said, "Our friend Lazarus has fallen asleep, but now I will go and wake him up." (12) The disciples said, "Lord, if he is sleeping, he will soon get better!" (13) They thought Jesus meant Lazarus was simply sleeping, but Jesus meant Lazarus had died. (14) So he told them plainly, "Lazarus is dead.
[1Th 4:13-14 NLT] (13) And now, dear brothers and sisters, we want you to know what will happen to the believers who have died so you will not grieve like people who have no hope. (14) For since we believe that Jesus died and was raised to life again, we also believe that when Jesus returns, God will bring back with him the believers who have died.
This was because since righteous Abel, all who died went to a holding cell referred to in Hebrew as “Sheol” but in Greek, hADES to await the day of judgment. So this is the picture:
Abel is murdered by his brother, Cain
he sleeps the sleep of death in hADES
Jesus descends as a “breath of life” to all in hADES and Abel is carried by angels to the Happy Place to accompany Jesus at his second coming
So death, sleep, resurrection.
But the “secret” of which Paul writes is that Jesus destroys hADES. The “sleep” step is eliminated. hADES is destroyed in the same fire that destroyed Jerusalem and the temple:
[Rev 20:14 NLT] (14) Then death and the grave were thrown into the lake of fire. This lake of fire is the second death.
So the “mystery”/secret of which he speaks is the fact that since the first century, sleeping in death/hADES is over with and the saints from that time on are immediately transformed. No more sleep nonsense.
[1Co 15:51-57 NKJV] (51) Behold, I tell you a mystery: We shall not all sleep, but we shall all be changed-- (52) in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised incorruptible, and we shall be changed. (53) For this corruptible must put on incorruption, and this mortal [must] put on immortality. (54) So when this corruptible has put on incorruption, and this mortal has put on immortality, then shall be brought to pass the saying that is written: "Death is swallowed up in victory." (55) "O Death, where [is] your sting? O Hades, where [is] your victory?" (56) The sting of death [is] sin, and the strength of sin [is] the law. (57) But thanks [be] to God, who gives us the victory through our Lord Jesus Christ.
And Paul says that not only won’t the saints any more have to sleep for thousands of years, they won’t have to sleep at all! And yet they will be changed/transformed.
For more information, please visit my Quora Space, Biblical Eschatology
The victory of 1 Corinthians 15 is not resurrection but rather the casting of hADES into the lake of fire so that the saints are immortal, no longer subject to death and not requiring resurrection. They pass immediately from this vale of tears into God’s presence because they have become immortal:
[Jhn 5:24 NKJV] (24) "Most assuredly, I say to you, he who hears My word and believes in Him who sent Me has everlasting life, and shall not come into judgment, but has passed from death into life.
[1Jo 3:14 NKJV] (14) We know that we have passed from death to life, because we love the brethren. He who does not love [his] brother abides in death.
[Rev 21:4 NKJV] (4) "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."
All of this was accomplished in the first century:
[1Co 15:50-57 NKJV] (50) Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; nor does corruption inherit incorruption. (51) Behold, I tell you a mystery: We shall not all sleep, but we shall all be changed-- (52) in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised incorruptible, and we shall be changed. (53) For this corruptible must put on incorruption, and this mortal [must] put on immortality. (54) So when this corruptible has put on incorruption, and this mortal has put on immortality, then shall be brought to pass the saying that is written: "Death is swallowed up in victory." (55) "O Death, where [is] your sting? O Hades, where [is] your victory?" (56) The sting of death [is] sin, and the strength of sin [is] the law. (57) But thanks [be] to God, who gives us the victory through our Lord Jesus Christ.
[2Ti 1:10 NASB95] [10] but now has been revealed by the appearing of our Savior Christ Jesus, who abolished death and brought life and immortality to light through the gospel,
[Heb 2:14-15 NASB95] [14] Therefore, since the children share in flesh and blood, He Himself likewise also partook of the same, that through death He might render powerless him who had the power of death, that is, the devil, [15] and might free those who through fear of death were subject to slavery all their lives.
[Jhn 11:23-26 NASB95] [23] Jesus said to her, "Your brother will rise again." [24] Martha said to Him, "I know that he will rise again in the resurrection on the last day." [25] Jesus said to her, "I am the resurrection and the life; he who believes in Me will live even if he dies, [26] and everyone who lives and believes in Me will never die. Do you believe this?"
Answer summary:
Prior to the abolition of death, the righteous dead sleeping in Hades had to have their old body resurrected and changed, but in the Kingdom Age (the current age) the saints are immediately changed.
