I have an Android app that uses Firebase Firestore database. I have successfully added a functionality (a different Class, activity) for the user to message me (the developer). Here it is:
package com.HidiStudios.BeST;

import android.app.AlertDialog;
import android.os.Bundle;
import android.text.TextWatcher;
import android.text.Editable;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import androidx.annotation.NonNull;


import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.OnSuccessListener;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.DocumentReference;

import java.util.HashMap;
import java.util.Map;

public class MessageMeActivity extends BaseDrawerActivity {

    private EditText messageEditText;
    private Button sendMessageButton;
    private FirebaseFirestore firestoreDB;
    private TextView charCountTextView;
    private static final int MAX_CHAR_COUNT = 2000;

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

        messageEditText = findViewById(R.id.messageEditText);
        charCountTextView = findViewById(R.id.charCountTextView);
        sendMessageButton = findViewById(R.id.sendMessageButton);

        initializeDrawer();

        // Initialize Firestore
        firestoreDB = FirebaseFirestore.getInstance();

        sendMessageButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                sendMessage();
            }
        });

        messageEditText.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence charSequence, int start, int count, int after) {
                // No action required before text changes
            }

            @Override
            public void onTextChanged(CharSequence charSequence, int start, int before, int count) {
                // No action required during text changes
            }

            @Override
            public void afterTextChanged(Editable editable) {
                int charCount = editable.length();
                String counterText = charCount + "/" + MAX_CHAR_COUNT + " characters";
                charCountTextView.setText(counterText);
            }
        });
    }

    private void sendMessage() {
        String messageText = messageEditText.getText().toString().trim();
        if (!messageText.isEmpty() && messageText.length() <= MAX_CHAR_COUNT) {
            Map<String, Object> message = new HashMap<>();
            message.put("content", messageText);

            firestoreDB.collection("messages")
                    .add(message)
                    .addOnSuccessListener(new OnSuccessListener<DocumentReference>() {
                        @Override
                        public void onSuccess(DocumentReference documentReference) {
                            messageEditText.setText("");
                            showSuccessDialog();
                        }
                    })
                    .addOnFailureListener(new OnFailureListener() {
                        @Override
                        public void onFailure(@NonNull Exception e) {
                            Toast.makeText(MessageMeActivity.this, "Failed to send message.", Toast.LENGTH_SHORT).show();
                        }
                    });
        }
    }

    private void showSuccessDialog() {
        // Using AlertDialog for Success Popup
        new AlertDialog.Builder(this)
                .setTitle("Message Sent Successfully")
                .setMessage("Thank you for your message.")
                .setPositiveButton("OK", null)
                .show();
    }
}


I want to be notified (as a developer) when a new message arrives. How can I get (for example an e-mail) notification, if someone has sent me a new message?