Fusing and buring of Helium core/shell
The helium core becomes significantly denser and hotter after a star has left the main sequence.
Providing the overall mass of the star is greater than about 50% that of the Sun, the helium core will become hot enough to begin fusion whilst the star is a red giant.
From there, the star will pass through phases of burning helium in its core, then hydrogen and helium in shells around an inert core of carbon and oxygen.