How are massive stars born?
Short answer is we aren't entirely sure currently. It's still a mystery.
As with most stars, the formation begins with an accretion disk in which gases and dust swirl around it. These gases and dust continue to rotate around the forming star. As the gases and dust rotate around the star, slowly they fall into the star itself. As they fall, they form into clumps with high density. These clumps serve as fuel for the young star.
This is how they are believed to be formed as of now.