Where does the serialization happen?

I'm trying to find the code that performs the SOAP serialization when we call a ClientBase method of a SvcUtil generated proxy. Which file(s) carry that code?

I have tried to generate the same SOAP envelope that the proxy creates by serializing the SvcUtil generated request classes using `DataContractSerializer`, `XmlSerializer` and `SoapFormatter` (3rd party NuGet package). However, none of them produce the correct SOAP request. So, I'm interested in seeing the code that produces the correct SOAP xml.