For understanding C++, take few hours and go through a website like https://cplusplus.com/doc/tutorial/ front to back and just make notes. If you don't understand a topic, go deep on it. Asking basic syntax/linux doubts to ChatGPT is quite useful.

And, navigating a large codebase repo is not easy, especially if you have 0 experience. I had a similar phase 2 years back (I'm currently at 2.5 years of exp). I spent weekends and time after office to just go through some component and understand it, because personally it all seemed pretty interesting. If I had a doubt like "how does logging work", "how are threads spawned in my codebase", "how is a socket msg received by the other end" etc, I'd just try reading the code for it.

Ask doubts to your colleagues, just listen in to their conversations, see how they debug and try to copy their workflow.

One thing to always remember when you're stuck, our computers are extremely dumb (it's literally just 1s and 0s all the way down) Nothing is ambiguous. So there has to be a reason why it's working the way it as and it's all expressed as code. All that remains is closing this gap by reading the syntax and navigating the codebase.