Juniors are a fun bunch to work with.
Over confident, hero complex of that fresh graduate high, and then thrown in to the real world! Where there hopes and dreams are crushed in minutes when they see what monolithic applications really look like!!

But don't let that overwhelm you, your not going to be changing all of it any time soon, hell some of this code hasn't been touched in 5+ years and still works without fail.

Don't stress about the work load, you can only write 1 line of code at a time anyway, and hell, even seniors make mistakes.

The key about being able to manage this beast is simple, break it! Because the more you break it, the more you'll understand how a project is put together, for better or worse. Learn from the examples in front of you, and learn what not to do in the future 😎

But more importantly, plan your changes, whiteboard the high level logic of what it is you want to add, then whiteboard in the current codebase and determine where to slice this bitch up, then when it all looks well and good, take out your scalpel and slice and dice time.
Don't worry, your changes aren't going to production anytime soon, hell, you'll be lucky to get past the first pull request with this working 100% the first time, and that's a good thing, learn from tour short comings and improve your own knowledge for the next time!