It’s easy! Don’t. It’s not possible to do.

Focus on one small area instead of the whole project. If there isn’t a “beginner” ticket selection then find one (or give yourself a goal). Figure out where that code is and start playing around with it.

As you branch out and work on more and more tickets you’ll gain more and more experience. You’ll understand how different blocks and systems interact and gain a better overall understanding of the code base in general but you’ll never be able to keep everything in your head. It’s just not needed.

And I don’t think it’s been said yet but as a former vi guy a good IDE was a huge boost to productivity. Ease of navigation around the code, intelligent searching, etc really helped out in the exploratory phase.