Definitely don’t spend much time learning C++ outside the scope of UE4, there’s too much stuff and tons of it isn’t relevant. Just google things you don’t understand as you come across them in UE4.
By far the best thing I’d say is to put a breakpoint somewhere in the engine code and just step through the entire game loop. And make sure you’re using a Debug build when doing this, otherwise stepping through the code won’t be very accurate (it’ll skip over code due to compiler optimizations).
Can also use ‘Find In Files’ (ctrl + shift + f) and search the engine for systems you’re interested in. This is also great for determining how to accomplish certain tasks, because there’s usually several examples already implemented elsewhere in the codebase.