For at least some versions of Spider-Man, it's implied that the knowledge and skill necessary to create the web fluid and web shooters in the first place was itself part of his powers - basically, the spider-ness he got from the bite included a sort of subconscious understanding of how to chemically create webbing like a spider would.
It also helps explain why he didn't bother to invent it beforehand (and thus make a ton of money, helping his family), and why none of the other super-geniuses in the Marvel universe have ever bothered inventing anything similar (considering all the incredibly valuable uses it could have for normal people).
you can't create something without an idea.
and webslinging would get normal people killed ---