To use configure your boot loader to invoke it or symlink it to /sbin/init. Also, you will need to use openrc-shutdown8, to halt, reboot or poweroff the system.
The default runlevel is read from the init command line, the rc_default_runlevel setting in rc.conf, the kernel command line, or it is assumed to be "default" if it is not set in any of these places.
doesn't manage getty's directly, so you will need to manage them another way. For example, you can use the agetty service script as described in agetty-guide.md in this distribution.