MySql 8 on RHEL 8 strange problem

mysqld can still be startup with default datadir=/var/lib/mysql even /etc/my.cnf not exists.

Checked the whole disk that there is no my.cnf in any other location.

I have been trouble shooting with mysql ignoring my definitions in my.cnf file.

No matter what I put there, systemctl can still start my mysqld process.

Eventually, I renamed my.cnf file to other filename meaning that there is no my.cnf file exist but systemctl can still start up mysqld process.

Anyone can help me to understand the possible reason?
Thanks a lot,
Charles