Right now mysql_install_db prints this as part of its output:

To start mysqld at boot time you have to copy

support-files/mysql.server to the right place for your system

Which is referring to the SysV init script. Most distributions nowadays use SystemD instead of SysV though, and so that message should at least also mention the also bundled SystemD mariadb.service file, or even produce different output depending on whether the systemctl binary is found on the system ...