Search notes:

/usr/lib/systemd/systemd

/usr/lib/systemd/systemd is likely the most important systemd binary.

Started as first process vs. subsequent starts

First process

In a systemd environment, systemd is started as first process and hence has PID 1.
This process then brings up and maintains userspace services.

Additional processes

Besides the systemd process with PID 1, additional systemd processes can be (or are?) started for users that are logging into the system.
If these additional processes are started via a (symlinked) init name, it will execute telinit (for compatiblity with SysV) and pass the command line arguments.

/sbin/init

Typically, /sbin/init is a symlink to /lib/systemd/systemd: 
$ readlink -f /sbin/init
/usr/lib/systemd/systemd

TODO

The user manager instances are started automatically through the user@.service(5) service.
man 8 telinit

Index