How can billions of people have the same dream at the same time?

Again, this question has no simple answer and is one of the reasons we were given A COURSE IN MIRACLES.  Simply put, only one entity, the Son of God, is dreaming this dream, and the billions of people (us — like a hologram) who seem to be separate individuals in the dream are images of Himself in His dream.  See also this Q and A, and the definition of Oneness.

“God’s Oneness and ours are not separate, because His Oneness encompasses ours.” (ACIM, T-8.V.3:1)