Look at the packages. Try to break it down into architectural layers. Understand in a broad sense what each layer adds to the one before. Rage that it wasn’t so much architected as cobbled together from pieces never designed to fit together. Decry it as total garbage and recommend total rewrite.

Pick a small bug or feature from the backlog and fix it. First iteration of a fix is probably shoehorned in there, then I try to adapt the fix to the code base. Matching the style and design of the code base is more important than my own preferences.

I’m a learn by doing kind of person.