why and how do humans exist when (eventually) we're all just gonna die, either by mass extinction or we just... die out?

The short answer is yes. The fossil record shows everything goes extinct, eventually. Almost all species that ever lived, over 99.9%, are extinct.

Humans have vulnerabilities. Large, warm-blooded animals like us don’t handle ecological disruptions well. Small, cold-blooded turtles and snakes can last months without food, so they survived. Big animals with fast metabolisms – tyrannosaurs, or humans – require lots of food, constantly.

So we’re vulnerable, but there are reasons to think humans are resistant to extinction, maybe uniquely so. We’re a deeply strange species – widespread, abundant, supremely adoptable – which all suggest we’ll stick around for a while