Do you find learning big codebases hard too?

I have tried several times to go into open source development but found very hard to make sense and learn about these big codebases fast. I suppose that devs who start working on existing projects with large codebases may have had this problem too.

Could you tell, what was your experience when starting working on a large codebase? Was it hard for you to make sense of this amount of code? How have you dealt with it?

Maybe, you can recommend some tools, methods, techniques to make learning easier, and becoming productive on the project faster. I was thinking about possible solutions, but haven't found anything very useful yet.