That's what I was thinking. My first job was dev on a C++ and Java project that was in the high hundreds of thousands of lines when I started, and grew from there. Client-side had about 5 core binaries, server side had about a dozen, and each of those was a sizeable file.

Starting to understand it involved reading our documentation on the data-flow between different components during operation, to know the purposes of the important binaries. For the really core components, we had a fair bit of documentation at the level of classes.

You'd usually end up learning the sections of particular programs that you worked on in great detail, the programs themselves as a whole in slightly less detail, getting fuzzier as you moved away from your areas of greatest experience.