init system

The init system is closely integrated with the kernel because it's what allows you to run services.

There's SystemD: an init system created by corporate developers as part of Red Hat (a now mostly unused proprietary form of Linux that is only really worked with by high-level programmers employed at large companies) and alternatives like runit and OpenRC.

In part both due to Red Hat's poor reputation and the fear of having a system that has access to essentially every major function of your computer SystemD tends to be viewed negatively by subsets of the community but ultimately SystemD is fast and handles pretty much everything well and in a predictable fashion including networking, sound, and display management.

It's also used on almost every distro and if you read through What does distro mean? you'll probably realize that most of the distro variants are actually just derivatives that either remove SystemD or that don't feature it to begin with.