There are certain instances in the Bible that were not witnessed. For example:

1. The Magnificat episode

2. Jesus' temptation in the desert

3. Tha agony in the garden (the apostles were sleeping).

So how did the Evangelists know what to write?

submitted anonomously

First of all the evangelists were not necessarily witnesses of all that is recorded in the Gospels and a lot of what they later wrote was a result of oral tradition, that is, the episodes and sayings that were passed on by word of mouth most of all from the apostles. We can assume that once the Jesus was alone on the Mount of Temptation and at certain moments all by Himself in Gethsemane then He Himself later told the disciples exactly what happened. The same applies for Our Lady and her Magnificat.

The most important thing to remember when reading the Bible is that even though God ‘employed’ human authors, He Himself remains the Divine Author who pans the whole Bible.

God’s purpose in giving us His Word is to reveal Himself and teach us His ways ‘All scripture is inspired by God and useful for refuting error, for guiding people’s lives and teaching them to be upright. 2 Tim. 3:16