Having worked on quite a few legacy applications in my career, I would say that, as for so many other issues in programming, the most important solution to this

Having worked on quite a few legacy applications in my career, I would say that, as for so many other issues in programming, the most important solution to this issue is good modularization of your code. That allows a new team to understand the application at high level in terms of modules interacting with each other, and when you need to make some changes, you only need to understand the modules involved, and ideally one at a time. So you don't need to form a detailed theory of the whole application all at the same time. What I'm finding with LLMs is that, if you follow good modularization principles and practices, then LLMs actually make it easier to start working on a codebase you don't know very well yet, because they can help you a lot in navigating, understanding "as much as you need", and do specific changes. But that's not something that LLMs do on their own, at least from my own experience - you still need a human to enforce good, consistent modularization.