I used to work on Visual Studio 20 years ago. At the time it was 30M lines of code with a history going back over a decade and about ~1k people working on it. Product cycles were measured in years. Time spent working on new features was measured in months, since no one understood how it all fit together we then had to spend years playing whack-a-mole with bugs, to stabilize it before shipping it. I regularly had to debug through large parts of the stack I'd never been in before. So did lots of other people.

Being able to work effectively in large amounts of unfamiliar code was an expectation for mid-level engineers (L62 if I recall correctly). Its not a new skill, but it is increasing in value again.