Well , there is one process that is the first process to start when Linux ( and all Unixes) start up after the kernel is loaded. This first process was originally called init. All other processes are child processes … either children of init directly, or children of children…
The init system became rather complicated with age and attempts were made by several distros to replace it. The winner at the moment is systemd ( ie system daemon) produced by Red Hat. There was an anormous controversy when Debian decided to adopt systemd, and Debian was split into Debian and Devuan. The other major distro to adopt systemd is Arch.
What the controversy was all about, was not so much replacing init , but the concern was the way in which Redhat implemented it. They expanded its function to include a whole lot of system activities other than being the first process and starting other processes.
The argument against that was that Unix has long had the philosophy of “One task one process” and " A program should do one thing and do it well" . That philosophy IS Unix. By expanding into a variety of functions systemd had violated this basic principle… and as a consequence exposed itself to being complicated and bug prone.
OK so what are the alternatives to systemd
Devuan offers choice ot the orininal init, OpenRC, and runit
Void is runit only
Gentoo defailts to OpenRC, but offers others
BSD mostly uses OpenRC
PS LinusOS uses original init
There are more but you get the picture. The Linux community is divided on this issue.
I just wish that more distros would be like Devuan and offer alternatives. It cant be all that difficult… Devuan can do it with a much smaller staff than Debian
My personal preference is for runit. It is ultra-simple… starts daemons and does nothing else… and the learning curve is half an hour. In contrast the learning curve for systemd is months or years… you might never remember all its commands.
Make your choices ( of distro) carefully. Some are more complicated to manage, and therefore more bug prone, than others.
Regards
Neville