"For example, Paul’s letter to Corrintians-was that an actual written document that existed during Paul’s life? and did he use it during preaching to other Christians?"
Since Paul couldn't be everywhere at once in his evangelical travels, he sent letters to the various churches to help them solve the specific problems they were having. These letters are so rich, theologically, that copies were customarily passed from church to church to spread the word. The priests at these various churches used the epistles as a theological basis for their teachings. Paul, being the source, obviously didn't need them in order to preach. After all, the infant churches shared problems in common--misunderstandings, heresies, personal conflicts, etc.--and Paul's exhortations and theological teachings had universal application. Even to our own day, in fact.
I might suggest reading Father Patrick Henry Reardon's excellent Daily Reflections
, which are available online.