One of the trickier things about C++ for me (in terms of “getting lost”) when I first started was that there’s no standard C++ project structure. It varies based on the build system you use, whether what you’re building is just an API or something else, and personal preference. I recommend looking at some open source C++ projects like Tensorflow (Google), LevelDB (also Google), folly (Facebook), MongoDB, and Thrill (a Spark clone in C++, https://github.com/thrill/thrill). Also pick your own projects you’re interested in. Take a look at basic tutorials for build systems like make, CMake, and Bazel. Hopefully that’ll give you an idea of what a modern C++ project actually looks like, and will help you start your own.

For books I’d like to add to the current top comment (may go away) API Design for C++. It’s a great reference on exactly what it sounds like.