Where does hydrogen fusion occur when there is no hydrogen left in the core?
When there is no hydrogen left in the core, fusion stops there. Under the force of gravity the inert helium core starts to shrink. The contraction of the core liberates gravitational energy which heats up the core and the hydrogen in the shell that’s surrounding it. Fusion then restarts in this shell. The shell fusion and the core contraction produce more energy than the star ever did as a main sequence star. As a result the star expands and becomes a red giant.
