I have an old .NET Framework 3.5 project from someone else that I need to get working again. The project has an old .sln file that I used to open it in Visual Studio 2022. The issue is that it uses personal libraries that were located somewhere else. I have downloaded these libraries and copied them to a folder inside the project but they only contain .cs files.

How do you properly add these .cs files so that the compiler will recognize the classes inside the libraries that were used throughout the project?

Here is a screenshot of how the folder structure looks: Project screenshot

I wish to add all the .cs files inside the BumTehnolink folder as libraries.