How to know where to start reading in a large codebase?

I've been coding a year and half and have written some webapps. I want to contribute to a large codebase but I'm a beginner and have no idea where to even start. How do I know what file or what folder to begin reading? Like let's say since I use react a lot and I want to contribute to react source code where would I begin? And how do I progress from there?