neowin.net/news/googles-code-base-spans-2-billion-lines-and-86-terabytes
This gigantic code base is made available to 95 percent of Google engineers, or about 25,000 people. With some 45,000 changes (commits) being made every day, and 15 million lines of code being modified each week.
Chris has talked about how programming code is reused, often for decades, often without the current coders knowing, or maybe even being familiar with what's actually in the collections of code they use. Google is likely one of the more extreme examples out there, but nonetheless the stats might help illustrate what Chris is talking about.
It's humanly impossible for one or a few people to know exactly what's in an actively developed, good sized code base -- by the time they went through & traced just a portion of it, odds are it's already been changed. That's one reason why conflicts &/or incompatibilities between software happens.