We know when we die saved now our spirit is instantly in the presence of the Lord. It wasn’t always like that. Prior to Jesus, the righteous who died were sent to paradise within the earth. They couldn’t go to heaven yet because, even in death, they still had their sin nature inherited from Adam. Sacrificing animals only covered sin temporarily, it didn’t remove sin. The sins of all men, past, present and future could only be forgiven permanently by the sacrificial death of a sinless human being. There was no such thing on this earth. Only God met the qualifications of being sinless. So God came off His throne and became flesh on the earth in the incarnation of Jesus Christ.
Jesus the man, while maintaining His deity, became that sacrifice that would remove the sins of every man, past, present and future. When Jesus died on the cross all the sins of those then in Paradise were instantly removed which allowed Jesus to go down there and take those saints on to heaven before the resurrection. We see an illustration of Paradise (and hell) in the parable about the rich man Lazarus and the beggar. The beggar died saved and went to Paradise within the earth, also known then as Abraham’s Bosom. Lazarus died and went to hell within the earth where he was in constant pain and torment.
The two sides could see each other but could not cross over. Think of these two areas in the earth as holding cells pending God’s final judgement. Paradise is no longer being used but the other side, often referred to as Sheol, Gehenna, hades, the pit or hell is still in use. All the unsaved dead, then and now, go straight to the hell within the earth where they wait in torment for the end of the millennium when they will be resurrected bodily to appear before the Great White Throne judgement and be judged by Jesus. Since they rejected God’s mercy and grace in life they will be sentenced to torment 24/7 for an eternity in the lake of fire along with Satan and his fallen angels. An angel of the Lord will physically lay hands on them and throw them into the lake of fire.