The biggest mistake engineers make when going into a new codebase is to excessively read through the code and documentation. This is a horrible way to learn.
The problem is exacerbated because the supporting documentation of the code is usually out-of-date: the docs can't keep up in a fast-moving tech company.
Instead, just dig in and start coding!
The key takeaway is that you should get your hands on the keyboard (even if it's uncomfortable).