Some seem to have the idea that the Bible is only a book for preachers to find a text in. I have known some preachers whose sermons showed that is what they thought. They selected a text and never got back to it or else paid no attention to it's context. Preachers have no monopoly on understanding the Bible.
Preachers gain a knowledge of the Bible in the same way as anyone else, through study.
It is a mistake to think of preachers as apostles. The apostles were inspired by the Holy Spirit to receive revelation. Since revelation has been completed we have no living inspired men today. We have an inspired book. The inspired prophets and apostles still function through the Word.