> I'm more efficient when I understand a codebase fully in my head

In any system of meaningful size, no single individual on the team will have intimate familiarity with the entire code base. You might understand the overall architecture, know the actual code of select areas very well, and can quickly dive into most areas and be proficient, but the volume of changes and number of people working in the code base puts a hard limit on how much you could possible know about every single line in the code.