Seeking general advice

I have a large C++/C project in work. Its a large scale messaging switch with 135k lines of business logic. Its basically a massive CLI with different CLIs around it. The CLIs around the main CLI are things to interact with it, like mini toolkits. Its very awkward to work with. We build these executables and then run them in command line and hook the debugger onto them from VS code. The whole thing runs in a docker container. We run everything inside WSL. I have about 6 years exp as a dev with numerous different technologies but never any commercial C, C++ or C#. The code isnt great at the best of times but what code is. The abstractions a bit all over the place from what I can understand and theres multiple threads doing god knows what. Testing is really difficult. We use GTest and its hard to work with. It doesn't spit out error messages when you run the tests. I dont understand C++ that well yet. We have a Spring/React service that pairs with it to do other things and I have been working on that recently. I have managed to near enough deliver a big feature. Now I want to try and get really hands on with the C++ side of it now. Whats the best way to tackle this ? I have some time off at the end of October so im thinking about trying to do some home project to learn more about C++. Does anyone have any recommendations for stuff I need to know ?

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