I think this is something of a strawman: there are plenty of million line (plus) systems and software suites, but I think million line programs are relatively u

I think this is something of a strawman: there are plenty of million line (plus) systems and software suites, but I think million line programs are relatively uncommon by comparison. Especially so if you don't count library code, and I don't because otherwise you end up doing things like counting the number of lines of code that back the Win32 APIs in your program line count, which doesn't seem sensible. Most of the time we treat libraries as black boxes, except on rare occasions, so don't tend to think of them as part of "our" codebase[1]. Most million line plus systems are broken down into subsystems (applications, services, libraries, etc.) that in isolation are smaller, and in many cases much smaller. Even with those rare million line of code monoliths, for the most part there's some level of organisation in the codebase: layers, subsystems, components, or a mixture of these and whatever else is appropriate. This organisation, and particularly when coupled with modern tooling, facilitates the understanding of even very large systems. I don't say that it's necessarily easy but, then again, neither is understanding a large, complex system implemented in K. [1] Except when thinking about security because third party code has been shown to be a rich source of vulnerabilities.