maybe think of it as "ash"
the newly fused element is more massive than the ones it came from
and sinks to the bottom (core)
When enough "ash" is accumulated (and the star is still big enough)
the previous ash begins fusing into some new heavier "ash"
rinse and repeat until you get to iron "ash" but no further.
the reason is; every fusion lighter than iron is exothermic
two atoms fuse into something bigger than either,
but smaller than both,
plus releasing some energy (yay).
when you get to fusing iron you need a ball about 20 miles in diameter
and fusing does not give up energy,
fusing iron needs to take energy from its environment.
good thing it is not in the middle of a star big enough to grow a twenty mile ball of iron.
and that kids is where supernova come from
once iron fusion starts it all goes in moments collapsing the core
and the star above where the core was "falls" into that space
gravitationally propelled with enough speed that when it hits the opposite side (falling just as fast in the other direction) everything fuses into "whatever"
that is about as exciting as it gets in this universe