It seems you are in confused state based on the question.
ReadyAPI has suite of tools to test the functionality / load / mock functionality of API.
SoapUI NG is the one of the product from the above suite which you need to use design your tests (in the from of project > test suite > test case > test steps). In case, need to know how create project, this helps.
Where as TestServer is concerted, it is just used to execute the tests; but not for creating scripts.
Here is excerpt from documentation:
ReadyAPI TestServer is a GUI-less application that runs on your local computer or on some other machine in your network and executes API tests. The topics of this section contain detailed information on using TestServer.
Hope the above clarifies your queries.
The above document link helps you to get started with test server as well.