EDIT: if its pure (not reactive to any other variable but other variables may react to it) they will auto memoize I guess to avoid their own reactivity engine doing a bunch of useless shit when no value was actually updated. Correct me here if I am wrong.
You have to opt-in to prop-diffing conditional re-renders (which I wouldn't call a "reactivity engine" either) per component, via React.memo.
And then you also have to opt-in to prop memoization for non-primitives for the prop-diffing not to be useless.
These re-renders might not result in any actual DOM changes since there is an additional vDOM diffing step from the resulting render (which, again, I wouldn't call a "reactivity engine").
