what happens if you implement an automatic property
public string Foobar { get; set; }
and then code the corresponding variable
private string foobar = string.Empty;
Will the automatic property use this variable or does the compiler generate an additional variable?
No, the automatic property will not use your variable. It would be just like any other field called foobar.
The name smilarity does not influence the compiler in any way.
The compiler will generate a field behind the scenes but you do not have access to the backing field of the automatic property in any way.
This post shows how things work at the IL (Intermediate Langauge, Assembly of C#) level.