decompile dll unity for vector3

I’m trying to figure out how it was made ​​the Vector3 class. How can I decompile the dll? I’m trying with “Il Spy” but which file should I open? sorry for the question but I just want to learn

In any of your scripts in MonoDevelop, right click on the word “Vector3” and select “Go to Declaration”. That will take you to the Assembly Browser where you can view all the non-internal implementations of the UnityEngine.dll