How is debugging a "micro level skill"? Jumping into an unknown project/system/whatever is something you will likely have to do at some point. (Sometimes it mig

How is debugging a "micro level skill"? Jumping into an unknown project/system/whatever is something you will likely have to do at some point. (Sometimes it might be code you wrote yourself years ago, but revisiting without any context makes it feel unknown.) Is this hypothetical guy always writing 100% correct code?