I think that John told Luke almost all the things he wrote in his Gospel. Let's look at what Luke tells us about the beginning of Jesus' life: the Annunciation, the Visitation, Bethlehem, the birth, the hidden life. Who actually witnessed the hidden life? Luke is a historian and he has a very sharp mind. He warns us in the Prologue to his Gospel, that he wants to carefully go over the whole story from the beginning (Lk 1:2-3). He wants to have witnesses. Who witnessed the very beginning? There was only one witness: Mary. And who took Mary into his home? John. It is therefore easy to understand that Luke received a lot from John. I'm not saying that he received everything from him, but John is still the main source. Moreover, if we had been in Luke's place, and intelligent like him, we would all have done the same. Let's take a moment to put ourselves in Luke's place: Mary, no doubt, was still living with John. Luke wants to write a more complete Gospel than Mark or Matthew. What should he do? Naturally, he went to the source. Intellectuals today know how to find old manuscripts. If they hear that an unusual old manuscript exists in some library, which nobody else knows about, they don't hesitate to travel miles and miles and waste a considerable amount of time trying to understand that manuscript. And what if the knowledge we are seeking is not in writing, but in a source that is so hidden, would we spare any means? Mary's heart is not a document, it's a source. "She stored up all these things in her heart" (Lk 2:19 & 51), and that was the Word of God.