Things left over from before the world went entirely to hell.
There were at least two or three centuries between our now, and that of the World War where everything went straight to hell; in the interveening time technology advanced, in most cases rather spectacularily, and even now, centuries later, there are remenants of what went before – power sources still running centuries after abandonment, fabrics that can sheild from even the most intense flames, and the occasional feral(or domestic) critter that just ain’t nautral.
Old Tech is Black Box technology; A really hardcore scholar(the equivalent of a self-taught multiple PhD) may have some vauge ideas how it works, but they’re not going to be sure, and they’re certainly not going to be able to build one in their basement. Mostly people are lucky if they can reliably turn it on/off, and keep it from exploding – which has been a problem in more than one case.
Just as a note: Some relatively advanced technology is still available – there are numerous individuals and groups who could, say, build a simple electric generator. The main limitation on such tech is that it needs to be constructed, from the ground up, by hand, usually by a single, resource strapped artisan.