Question about Retrofit - why is it implemented like that?

Question

Android

Here is an example of a simple class for working with Retrofit:

public class NetworkService {
    private static NetworkService mInstance;
    private static final String BASE_URL = "https://jsonplaceholder.typicode.com";
    private Retrofit mRetrofit;
 
    private NetworkService() {
        mRetrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();
    }
 
    public static NetworkService getInstance() {
        if (mInstance == null) {
            mInstance = new NetworkService();
        }
        return mInstance;
    }
    
    public JSONPlaceHolderApi getJSONApi() {
        return mRetrofit.create(JSONPlaceHolderApi.class);
    }
}

My main question is, why call every time create(JSONPlaceHolderApi.class)in the method getJSONApi()? Why create(JSONPlaceHolderApi.class)is it not called once (for example, in the constructor), but each time it will be called when calling getJSONApi ()?
And the second question - if I create several interfaces (JSONPlaceHolderApi, JSONPlaceHolderApi2, JSONPlaceHolderApi3, etc.), then I need to work with them through the same Retrofit instance, similar to how it is implemented for the interface in the example JSONPlaceHolderApi?
Thanks in advance.