I'm an atheist, but I can answer you.
First, you have some vague prophecies written in the Old Testament.
A long time afterwards, you have a fellow born who gets a meager following. Later he dies because he made the wrong people angry.
Decades after this guy's death, that meager following has a meager following, but a slightly larger one.
This is where the New Testament enters the picture. Some people, who are very familiar with the "old prophecies" and who like the stories of this guy who died a long time ago, write their own stories. These stories are written to convince people that this guy who died fulfilled the older prophecies. The earliest writer of these stories is Paul, who didn't really give much about the history of Jesus or much of his teachings.
Much much later, the Gospel of Mark is written. It includes even more made up stuff about that dead guy's early childhood, more of his supposed teachings and so on. Other writers, like Matthew, Luke, and John, among others, also take Mark's writings and other stuff they've read, and make up more Gospels.
Much much much later, a group of Bishops figure four is a good number, and trim off the number of Gospels to just the ones from Mark, Matthew, John, and Luke. The rest become apocrypha. Along the way, much of those older texts is edited and re-edited. For example, there have been three endings found for the Gospel of Mark, which was the earliest of the Gospel writers.
That is how Messianic "prophecies" got fulfilled. They weren't actually fulfilled, they were just written that way.