How is the multi-stage fusion process in a Red Supergiant internally structured?

In shells surrounding the inert core, like layers of an onion.

The internal structure of a massive star evolving into a red supergiant is highly complex and characterized by successive layers of nuclear burning, often compared structurally to the layers of an onion. Because these stars achieve enormous central pressures, they can maintain fusion through multiple elements successively. Once one fuel element is exhausted in the center, the core contracts, heats up, and ignites the next heavier element in a shell immediately surrounding it. For example, after core hydrogen is converted to helium, helium fusion begins in the core while a shell of hydrogen continues to burn around that. This nesting process continues, building inward through fusion stages like hydrogen to helium, helium to carbon and oxygen, and sometimes progressing all the way up to silicon fusion, which ultimately produces the iron core that signals the end of the star's life.