Hey, @alexdo

1) I tried adding these lines of code in wp-config.php thus it made the domain configuration in the Wordpress admin panel locked.

define( 'WP_HOME', 'http://example.com' );
define( 'WP_SITEURL', 'http://example.com' );

2) So again I updated the database using these commands in the server terminal.

mysql> UPDATE wp_options
    -> SET option_value = 'example.com'
    -> WHERE option_name = 'home';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> UPDATE wp_options
    -> SET option_value = 'example.com'
    -> WHERE option_name = 'siteurl';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

3) I even tried git cloning var/www/html into my local machine to see if there were any codes edited in a hardcoded way, but nothing seemed to be edited as blog.example.com.

4) It just kept showing me blog.example.com and www.blog.example.com when I tried certbot --apache again. I tried rebooting the droplet too. So finally, I destroyed that droplet and build a new One-Click Wordpress droplet again.

It would be really cool if you could explain to me what is going on behind there I was missing out on.

Yes, I would like to know more about this. If that is correct you can also alter the Apache configuration file and change the domain from blog.example.com to example.com in the server block as well.

Thanks for this suggestion. I will keep that in mind next time. I will recommend you to create a snapshot of your droplet and also backup your data in order to have a working copy of your site (both files and database) in case something goes wrong.