I've spend my career reading code. I find it particularly difficult without guidance. I would recommend using some old school technology (pen/pencil and blank paper) and draw diagrams, write down every question and observation, notes. Others may recommend using some digital tool, but be mindful how much energy you spend organising your thoughts, drawing with a digital tool. If your goal is to understand a software, then don't waste time creating beautiful documentation first.
It is also important to have a clear goal. Let's say you want to understand how webpack starts up, or how it does a specific feature. Make this question your main concern and don't wander around. Don't try to understand everything at once. Divide and conquer.
It is also useful if you can ask questions, but please spend some time coming up with a theory first why things are the way they are. Formulate a hypothesis (e.g. "this piece is necessary because it handles an edge case", or "this piece of code looks uglier than the rest, is there a reason?") and try to prove it. If you can engage with the community, the better, but please don't outsource your "thinking efforts" to other project members. You can't learn how to reason about code, if you don't reason.
The most important one: be humble. Just because you would've solved a problem differently, it doesn't mean that the code is bad. Don't spend time judging the code. You are there to learn from others, so be open to other solutions. Whether it is a good or bad example is so difficult to judge... be patient and you will realise what kind of code is easy to understand. Once you have some idea, take that knowledge to your next project. This may take weeks/months/years depending on the project size.