You can easily tell if your system has runit… process #1 is called runit instead of init.
There is lots of info on runit here
There is wikipedia page on init systems
It includes a list of all systems, and there is some info on scripts and on the history of init systems.
My experience is that sysVinit scripts are rather complicated and are launched by sets of symbolic links called runlevels. There are six runlevels. The sysVinit links are in /etc/rc*.d and the scripts are in /etc/init.d
Runit scripts are simple. They too are launched by links but there is only one set of links and there are only 3 runlevels called start, run, and stop. Runit’s links are usually in /etc/service and its scripts are in /etc/sv
I do not know how systemd does it. Does it use a link farm?
I dont think we have heard the last word on init systems yet.
They will continue to evolve, until we get it right.
What OpenSUSE does next will be a good pointer to the future
direction. They are more levelheaded than Debian.