SystemD is the manager of everything, and it is used to manage everything on
most Linux distributions, but some don't, including the Windows subsystem for Linux or wsl.
If systemd is used, it's the first thing that is started after starting the Linux kernel, and the
main function is to manage services, but it also manages mounts, timers, patch hardware, and so much more.
Amongst the key features of systemd is that it is event driven, and that means that it can
react to specific events and make sure that specific stuff is only loaded when it is really required.
The items that are managed by systemd are known as units, and you should focus
on learning how to manage services. With systemd it can do so much more, but
for the basis of Linux, it's important to know how you work with services.