Ask HN: How do you navigate long, complicated code?

In my job, I have to work with existing code that is thousands of lines long per class, with tons of different classes. It's hard for me to memorize what every single class and function does just because there are so many.

Ideally, I would like to memorize what everything does so that I have a big-picture understanding of all of our code, and can approach problems from this perspective.

But, to get to this point would take hours and hours of reading the code over and over.

I wanted to ask other developers, what is your strategy in dealing with lots of code like this? Do you approach it from the perspective of trying to know everything, or do you just deal with the small blocks that are most relevant? If the latter, how do you think about how this small block affects everything?