Are we looking at the original code in a Disassembler or reverse engineered CIL?

Question

Take ILSPy. When I view my assembly am I looking at my original C#? Or, is this code reconstructed from CIL using some type of reverse engineering process?

My understanding is that release assemblies do not include any original code, just CIL. So, does it make a difference if I build my assembly in release mode?