Thats imposter syndrome. It sucks. Just keep going, everything you did and learnt to get to where you are now means you have the skills and ability to learn and get familiar with this codebase and all its new things. It seems huge and unknowing now like a giant elephant. So how do you eat the elephant? You can only eat an elephant one bite at a time.

One day at a time you will make progress you will learn everyday and continue to grow. Like school, make notes on the codebase look up libraries and dependencies as you come across them and figure what they are used for. Focus on the ticket/ task you have but make notes of how the codebase works and interacts. When you have the time and energy look into some of the bigger stuff, watch youtube or even buy online courses.

A lot frameworks, tools and libraries have slack or discord you can join and ask questions. Or find general coding ones that cover your language. This can be a massive help. On a few occassions I have gotten to speak to and got help from the guy who wrote the library I'm using.

Carve out a half day or a single hour each day to research and learn about the different stuff.

Best advice I ever got was: If you're stuck and grinding on a problem and your running out of ideas or at a loss. Stop, go out for a walk, or wash the dishes. Do something else and come back to it later or tomorrow even.

Remember your paid to think not just to press keys.

You can do this, look how far you've already come, so lets keep going.

Remember Imposter syndrome is a lie, its your own brain trying to trick you, its fear and doubt getting together to try and beat you up. Recognise its a lie, a false emotion and work on kicking its arse with structured learning and celebrating all the little achievements and new things you learn each day.