Never, Forever vs An Eternity: All The Same Concept?

Just wondering if there is a finite ending - obviously “never” means something will not happen, and forever implies the situation will never change.
The word “eternity” is the one that is iffy to me.

I have read where people try to define eternity with some really, really abstract definition that seems to imply there is at least a miniscule chance that it could end.

For example, somewhere I read that “eternity is the time it would take for a butterfly to destroy a solid lead sphere, the size of the earth, flapping only its wings against it.”
Granted, might take a couple of gazillion years, but I would guess that even that butterfly would get the job done, given enough time.

Are all of the words interchangeable - meaning ain’t ever gonna happen - or does “eternity” have some abstract finality that implies an end could come?

I am not particularly looking for dictionary definitions; what is your humble opinion on eternity vs never/forever?

“Eternity” isn’t about the duration: Something could be eternal but only extend across a few seconds. What eternal means is timelessness: To an eternal thing, there is no “before” or “after”. God is typically described as eternal: He sees all of time at once, as a single thing. On a less metaphysical note, photons are also eternal, in that no time passes for a photon between its emission and absorption.