Services which are available over SOAP will have a WSDL (Web Services Description Language) file. This describes the interface in a machine-readable format, allowing creation of a client with very little additional coding. SOAP clients such as Apache CXF (Java) and Zeep (Python) can consume this WSDL and create a native-language object with all the method calls automatically.
When the WSDL changes and additional method calls are made available, clients can update themselves to use the new method calls by simply updating the WSDL used by their code.
SOAP clients will also create the appropriate XML required for method calls, make the HTTP call then parse the XML returned.