Summary: In the graves Jesus preaches.
Jesus is crucified Friday, on a cross and buried by Joseph of Arimathaea (Matthew 27:57-60; Mark 15:45-46; Luke 23:50-53 and John 19:38). All four Gospel writers agree that it is Joseph of Arimathaea who buries Jesus. We have no doubt that Jesus is in the grave, but what is he doing? Is he resting from his labors? No, Jesus is in the earth preaching the Good News.
In the first letter of Peter, Chapter 3, verses 18-20 we read that Christ suffers on the cross for the sins of humanity. Christ, the Innocent, suffers for humanity, the guilty to bring humanity to God. On the cross Christ dies in the flesh, but God keeps his spirit alive. The spirit of Christ preaches the gospel to disobedient spirits of dead humans who live in the prison of death.
The spirits of those who are dead in the flesh live, just as the spirit of Jesus lives. Jesus is not in the grave preaching to the spirits of all the dead. He only preaches to the spirits of the disobedient people who live in the days of Noah. While Noah builds the ark God patiently waits for the people to repent but they refuse, and so only eight souls save from drowning in the flood.
The end comes when the living and the dead face the Judge to give an account of their lifestyle ( I Peter 4:5). This is why Jesus, dead in the flesh, but alive in spirit preaches to the disobedience spirits in the grave. These disobedient spirits died in the flood because they would not repent of sins. They died for their sins, but now Jesus dies for the sins of humanity which includes disobedient spirits who died in the flood. Jesus preaches to disobedient spirits so that they repent and live in the spirit with God eternally (I Peter 4:6).