Depends on the extinction event. There are plenty of scenarios (well not plenty, but enough) where most people die and supply chains are wiped out, but there are still libraries and warehouses full of shit (home depots, Amazon warehouses, and so on). Pharmacies would be good for raiding, too. Also farmer bob and his family died, but all of his shit is still there on his farm ready to be taken.

And you can get generators with three fuel sources, not to mention that solar panels are readily available and generally easy to put together for a janky half-assed set up, so anything that needs electricity can still be used. (Did somebody say power tools? Did somebody say well pump? Did somebody say small flow treatment system? Did somebody say refrigeration?)

And preppers (not all, but likely many) do amass survival books, encyclopedias and "medicine for dummies" books, and any of them that are serious are practicing Bushcraft enough to have a working knowledge of that.

I'm guessing the bunker is actually there to ride out the people killing other people to take their resources phase. Maybe a nuclear bomb?

Myself, I'm a half-assed homesteader, which is like being a prepper without all of the white supremacy stuff that goes with it. If there were an extinction event, and I somehow lived through it, I think I would last a couple more months than your average joe, but I doubt I would live into old age.