How can I fix a screw that has been stripped
Damaged screw-head

You can repair many small metal parts using a micro welder. Alternatively (and probably far more commonly) some people use a vice and a hammer to form the damaged head into a suitable shape for the next stage ...

You can re-cut a drive slot using a suitable tool. For slotted screws this would be a file intended for that purpose

Damaged screw-thread

For machine bolts, threads can be repaired using dies or helical inserts. I'm not aware of anything equivalent for things like wood-screws.

Standard practise

I admit I can't imagine any circumstances where I would not just drive to the store and buy a new screw (and probably a better screwdriver to match).