[noicebrewery]

THIS. Please ask your coworkers for help. If the company isn't a toxic shithole your colleagues will be happy to help out. Modern screen sharing, code pairing and mobbing tools have made doing this in a remote working environment incredibly easy, almost more so than in meatspace.

As for the massive codebase, try to specialise in an area and own it. Take lots of tickets around that module / namespace / whatever and you'll end up in a niche with lots of ideas about how to modernise, optimise or otherwise improve that code base, as well as teach you a lot about the companies coding standards and practices when you're ready to move horizontally into other parts of the code base. You will become trusted as an expert in that area and valued by your colleagues.

Large code bases are a reality in programming. All of your smaller app and websites are built on top of the massive code bases of your favourite JS frameworks, web servers and web browsers, which are built in operating systems that also have massive code bases. All of those code bases, by the way, started out small as well, and grew to meet bigger and more demanding requirements.

You are kneecapping your potential if you decide to stop at the software development equivalent of barre chords.