I've used libraries before, where I call the API surface that they expose based on method names and parameter types, without reading all the source. Generally those libraries don't implement my software's entire problem domain area; they tend to implement things like "CSV parser" or "HTTP server". The important code, that I'm actively reading and writing, tends to be the code around those library calls.

This is different from building a product, which you only interact with via UI buttons/CLI/etc, without reading any code to understand how it conceptualizes that product's problem domain area.

People do that latter thing, and we call them "users", not "developers".