My regular way to go would be to go bug fixing, the more bugs you fix the more you'd be familiar with the source code, this is what most open source projects do

My regular way to go would be to go bug fixing, the more bugs you fix the more you'd be familiar with the source code, this is what most open source projects do, but code quality is better so it takes less time to get familiar with it. Then if I don't have any bugs to fix, it would be to get very familiar with the application and read the logs on every action you do, take note of where they say they are and study and go on rabbitholes. Whatever you do, don't try to rewrite it, they almost never work unless you have a way too much time in hand.