If you've successfully compiled the C++ items into a single library file (DLL), you can reference the DLL in another project in Visual Studio using the Add Reference Option.
Be warned however, by doing so you're now mixing different 'technologies', and may run into issues. You may find it difficult to use the library you've just referenced, as you can see in this other question.
I would recommend to actually stick with C++, since the original files you started with in this library are C++. You can easily program C++ in Visual Studio as well (just create C++ project that's appropriate to your solution). You can also create a separate project in your solution to house these other files you've created externally, and VS will build/compile them for you as well.
In summary, try creating a new solution in Visual Studio with two projects:
- A C++ library project to house your C++ files already made
- A C++ application (of whatever type) that will use the C++ library just made.
If you follow my recommendation, you can use the Add Reference option previously stated above to add the library reference (#1) to the application project (#2) as a "Project Reference."
Regardless, you may want to take some free courses on C++ and Visual Studio to get you more familiar with the environment. Pluralsight, Lynda, are just a few I can name off the top of my head. Good luck!