What process within a massive star creates the onion-like layering of different burning shells?

Answer

Compression leading to ignition of heavier elements after hydrogen is gone

High-mass stars have enough gravitational force to compress the core repeatedly, igniting successively heavier elements like carbon, neon, oxygen, and silicon in distinct shells.

What process within a massive star creates the onion-like layering of different burning shells?
forcegravitystarpressure