How to use beginTransaction.hide().show() instead of beginTransaction().replace() on fragments in android studio?

Question

It has been only a couple days since I started learning android studio and today was the day I stumbled upon fragments. I am using the beginTransaction.replace() method to replace the fragment but this just create a new fragment every time. I have implemented a bottomnavigationview with fragments for each of the corresponding items in the menu. Here is my code:

package com.example.application;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import android.os.Bundle;
import android.view.MenuItem;
import android.view.View;
import com.google.android.material.bottomnavigation.BottomNavigationView;

public class Application extends AppCompatActivity {

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_application);
        BottomNavigationView bottomNav = findViewById(R.id.bottomNavigationView);
        bottomNav.setOnNavigationItemSelectedListener(navListener);
}

private BottomNavigationView.OnNavigationItemSelectedListener navListener = new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            Fragment selectedFragment = null;

            switch (item.getItemId()) {
                case R.id.firstPage:
                    selectedFragment = new firstFragment();
                    break;
                case R.id.secondPage:
                    selectedFragment = new secondFragment();
                    break;
                case R.id.thirdPage:
                    selectedFragment = new thirdFragment();
                    break;
                case R.id.fourthPage:
                    selectedFragment = new fourthFragment();
                    break;
                case R.id.fifthPage:
                    selectedFragment = new fifthFragment();
                    break;
            }
            getSupportFragmentManager().beginTransaction().replace(R.id.fragment, selectedFragment).commit();

            return true;
        }
    };
}

The replace() method is bad because I need to save states for each fragments. I don't want to use onSaveInstanceState methods to save states because they are just frustrating to work with. What I need is to the replace the code:

getSupportFragmentManager().beginTransaction().replace(R.id.fragment, selectedFragment).commit();

with replacing the replace method with hide and show. I need to hide the last selected fragment and show the currently selected fragment. How to achieve this?
And also I would like to know how big apps like Facebook, Instagram retain their states even though users scroll down on thousands of posts. Hope you will answer. Regards.