Right, first thing to get straight is that vs code doesn't open sln/csproj files. As KonajuGames states it only opens folders.
If however the folder you open os the root of your project that contains an sln, then it will read that to get the list of libraries available to do basic intellisense, code peek and docs.
VS code also can't compile or debug regular c# projects (although is does have some basic web debugging for c# / php). It is at its core, a functional text editor with syntax highlighting and git integration
Hope this helps