stash@{0} is the most recent (newest) stash, stash@{1} is "one older", stash@{2} is even older. The oldest stash is the stash with the highest number.

That's how Git defines it, there is no "how"; Quote from the docs

e.g. stash@{0} is the most recently created stash, stash@{1} is the one before it, stash@{2.hours.ago} is also possible

stash@{0} is the most recent (newest) stash, stash@{1} is "one older", stash@{2} is even older. The oldest stash is the stash with the highest number.

That's how Git defines it, there is no "how"; Quote from the docs

stash@{0} is the most recently created stash, stash@{1} is the one before it, stash@{2.hours.ago} is also possible