> you would learn how things work and then write the code

In a legacy codebase this may require learning a lot of things about how things work just to make small changes, which may be much less efficient.

I might still be naive about the industry, but if you don't know how the legacy codebase works, you might either delegate the change to someone else in the company who does, or, if there is no one left, use this opportunity to become the person who knows at least something about it.