Fusing and buring of Helium core/shell In the book &quot;Horizons_ exploring the universe-Cengage learning (2018)&quot;, p196, it states that

The nuclear reactions in a main-sequence star’s core fuse hydrogen to produce helium. Because main-sequence stellar cores are
cooler than 100,000,000 K, the helium can’t overcome the Coulomb barrier to fuse in nuclear reactions, so it accumulates at the
star’s center like ashes in a fireplace.

But if the temperature is not high enough to fuse helium, then how does the ending stage of a main sequence star is by burning its helium shell? Is fusing different from burning? If so, why doesn't a helium core star burning its core, even if it cannot reach the temperature about the Coulomb barrier?
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.