Android-RxJava: Update UI from background thread using observable

I just started building a simple Android app, in which I'd like to make a network request in a background thread and then update the main thread (UI thread) with the servers response. So far I used AsyncTasks, but future implementations I'd like to use reactive Java (RxJava). I have never done reactive calls before, so I'd like to have a simple but complete example (Observable and Observer creation and subscription) upon which it is possible to further build on.

I managed to include the RxJava dependency into the basic Android project and have written a very simple main activity using AsyncTasks for the network request. Now I tried to substitute the AsyncTask implementation with a reactive one, but got stuck in all the information regarding the Observable and Observer. I'm just not sure what exactly is necessary for a minimum but fully working example.

I'd really apprechiate a bit of help in transforming the main parts into an reactive implementation, since I don't know how to handle the generation of the Observable from the response string and subscribe an Observer.

Thanks.

package com.example.reactiveTest;

import androidx.appcompat.app.AppCompatActivity;

import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    private Button btnSend = null;
    private TextView result = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        this.btnSend = findViewById(R.id.button_send);
        this.result = findViewById(R.id.result);
    }

    public void onClickBtnSend(View view) {
        new SomeTask().execute("Just some String");
    }

    class SomeTask extends AsyncTask<String, Void, String> {

        @Override
        protected String doInBackground(String... strings) {
            // server request returning response String
            return response;
        }

        @Override
        protected void onPostExecute(String string) {
            // update UI with response String
            result.setText(string);
        }
    }
}