Starting work in a large, existing system is very much like trying to find your way around a very dark and very cluttered warehouse.
When you start, your flashlight is very small (if you even have a flashlight). You can only see tiny pieces of the room at a time. Nothing really makes sense and you have no sense of how any of the piles of clutter are related to each other.
After spending some time poking around the warehouse, you get to upgrade your flashlight. You can start to see more swaths of the room and once and can even infer a little bit about how different piles work together. But this is where it gets tricky. It's easy to make false assumptions about the parts you're seeing when the reality is still hidden in the darkness.
And the process continues like this as time goes by.
You get bigger flashlights and can see more of the room.
Spend enough time and you may even get to turn on some overhead lights.
If the system isn't too huge, you may someday turn on all the lights and can see the whole room at once. But you may never get to that point if the system is massive enough.
You may have a section of the room that you're most familiar with while someone else may know about other parts. This is great because then you can ask each other about what is in each pile of clutter.
In that case, you and everyone else in the room has to rely on each other.