In defense of not understanding your codebase

As a software engineer, how well do you have to understand your own codebase? My guess is that people who work on small codebases with low-turnover teams (say, Redis or games like The Witness ) would say “obviously you have to understand it completely, otherwise you can’t do good work”. I’d also guess that people who work on large codebases with high-turnover teams (say, the Google web search backend or GitHub) would say “obviously you can’t understand it completely, you just have to do the best you can in