What is the spiritual body that St. Paul talks about in 1 Corinthians 15:44?

Throughout the scriptures it is evident that there is no "immaterial soul" separate and distinct from the body. The soul dies when the body dies. Even of Jesus Christ it is written that "he poured out his soul to the very death." His soul was in Sheol, the common grave of all mankind. He had no existence as a soul or person during that time. (Isaiah 53:12; Acts 2:27; Ezekiel 18:4)

Consequently, in the resurrection there is no joining again of soul and body. However, whether spiritual or earthly, the individual must have a body or organism, for all persons, heavenly or earthly, possess bodies. To be again a person, one who has died would have to have a body, either a physical or a spiritual body. The Bible says: "If there is a physical body, there is also a spiritual one."

So the spiritual body Paul is talking about, refers to the body a being has and has to have, to live in heaven.

The apostle Paul is referring to the same type of bodies that both God and Christ have. These are bodies that are beyond death, corruption and destruction. They are bodies that don't need to depend on something from the outside to sustain them. Anointed followers of Jesus Christ leave their earthly corpses behind and receive these spiritual ones once they die and go to heaven.

He's referring to resurrected bodies as being spiritual, as opposed to the natural bodies we have now.  He's using planting as a metaphor for death, burial, and resurrection.

The meaning is clearer if we use the term "nature", rather than "body", which some versions of the Bible do. Humans are composed of a combination of a physical nature and a spiritual nature.

Same body Jesus Christ had when he was raised from the dead.

Paul  one of the 144000 to serve with Christ in heaven would be raised as a spirit creature because flesh and blood cannot go to heaven mentioned in verse 50-53

A person doesnt have a soul at death Ezek 18;4 so Paul would become an invisible  spirit creature to serve eternally as king and priest over the billions ressuerected to live on earth eternally Acts 24;15

it is a body that never decays never grows old it is a body that last forever. Thanks for the question.

The same one Paul describes in Philippians 3:21.