Extending \CodeIgniter\Database\MySQLi\Builder possible?

Have you tried extending the core class like this? : https://codeigniter4.github.io/userguide...asses.html

Yes, I put a file Builder.php in app/Database/MySQLi directory, extending \CodeIgniter\Database\MySQLi\Builder. I have also tried putting it in app/Library. Should I have put it elsewhere?