Android Studio – Android Push Notification even the app is fully ended

I’m really new to java, and I want to make a reminder app, and I am currently stuck on sending a notification to the user when user’s app is fully closed (Destroyed). Please help me, would be a much appreciate

My MainActivity

package com.example.notificationtest;

import android.Manifest;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.graphics.Color;
import android.os.Build;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import androidx.activity.EdgeToEdge;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.app.NotificationCompat;
import androidx.core.content.ContextCompat;
import androidx.core.content.PackageManagerCompat;
import androidx.core.graphics.Insets;
import androidx.core.view.ViewCompat;
import androidx.core.view.WindowInsetsCompat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.LocalDateTime;
import java.util.Calendar;
import java.util.Date;


public class MainActivity extends AppCompatActivity {
    Button button;
    String medicineName = "Medicine";


    
    String dayTime = "Mon, 13:27:00";
    Date timeSet;
    Date date1;
    Boolean running = true;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        EdgeToEdge.enable(this);
        setContentView(R.layout.activity_main);
        button = findViewById(R.id.notify);

        // Checking the Version
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
            // Check if permission is enabled or not
            if (ContextCompat.checkSelfPermission(MainActivity.this,
                    Manifest.permission.POST_NOTIFICATIONS) !=
                    PackageManager.PERMISSION_GRANTED) {
                // If Permission is NOT Granted Ask for permission
                ActivityCompat.requestPermissions(MainActivity.this,
                        new String[]{Manifest.permission.POST_NOTIFICATIONS}, 101);
            }
        }

        // Formatter
        SimpleDateFormat dayformatter = new SimpleDateFormat("EEEE, HH:mm:ss");

        try {
            timeSet = dayformatter.parse(dayTime);
        } catch (ParseException e) {
            throw new RuntimeException(e);
        }

        Log.d("Date Formatter", "" + timeSet + " n" + date1);

        
    }
    
    // Notification Function
    public void makeNotification() {
        String chanelID = "CHANNEL_ID_NOTIFICATION";
        NotificationCompat.Builder builder = new NotificationCompat.Builder(getApplicationContext(), chanelID);
        builder.setSmallIcon(R.drawable.ic_access_alarm_24)
                .setContentTitle("Medicine Reminder")
                .setContentText(String.format("Hey! It's time to take your medicine! %s", medicineName))
                .setAutoCancel(true)
                .setPriority(NotificationCompat.PRIORITY_DEFAULT);


        Intent intent = new Intent(getApplicationContext(), NotificationActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        intent.putExtra("data", medicineName);
        
        // When notification is clicked
        PendingIntent pendingIntent = PendingIntent.getActivity(getApplicationContext(),
                0, intent, PendingIntent.FLAG_MUTABLE);
        builder.setContentIntent(pendingIntent);
        NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        
        // When the android version is higher than OREO, Set the Channel ID
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            NotificationChannel notificationChannel = notificationManager.getNotificationChannel(chanelID);
            if (notificationChannel == null) {
                int importance = NotificationManager.IMPORTANCE_HIGH;
                notificationChannel = new NotificationChannel(chanelID, "Some Description", importance);
                notificationChannel.setLightColor(Color.GREEN);
                notificationChannel.enableVibration(true);
                notificationManager.createNotificationChannel(notificationChannel);
            }
        }

        if(date1.after(timeSet)) {
            Log.d("user alarm", "Ring");
            notificationManager.notify(0, builder.build());
            running = false;
        }
    }


}

• Make the code send the user notification even though app is closed
• Can you also explain it how it works easily please