Ther historical knowledge of the Church regarding Mary started with the Apostles and the Evangleists. Scripture tells us that Mary lived with St. John the Evangelist. St. Luke somehow gained knowledge of her youth, one that had no public witness, including the ability to cite verbatim her Magnificat prayer (Luke 1:46-55). This indicates the St. Luke received that knowledge, most likely, from Mary directly, or perhaps from someone closely associated with her.
Mary living with John and her reply to Elizabeth are found in Luke and are available to all. It is not special to the Catholic Church.