How to understand large codebase?

Hi everyone, I’m having a hard time switching between different projects at work.

I’ve recently been assigned to a new project that I’m not familiar with, and I’m struggling to figure out where to start. The tasks and issues I receive are often connected to other functions or modules.

The problem is that when I try to understand one function, I realize that I need to understand another function first. Then, to understand that function, I need to understand another one, and it starts to feel like an endless loop.

I’m not trying to understand the entire project at once, but I’m having trouble knowing which parts are actually important for the task I’m working on.

Do you have any tips or strategies for understanding a large, unfamiliar codebase and tracing how different parts of the system work together?

For context:

I work mainly with Spring Boot.

I have functional requirement documentation provided by the Business Analyst.

The project is fairly large, with different functions and modules depending on each other.

I’d really appreciate any advice on how experienced developers approach this situation, especially how you decide where to start and how far you should trace dependencies before making a change.