It used to be process 1 was init, and it forked/exec'd everything else typically from /etc/rc (this elides over a lot) -its the first binary the kernel runs. Everything else stems from the birth of user process number 1.

Fork/exec is the lightweight process inheriting of open i/o and memory state with copy-on-write which made Unix a joy to work with. It's a delightful mapping of kernel and userspace into complex inherited tree of processes sharing that origin state (or not. You can close it all down, change things.)

systemd is a complex mega binary which parses control files and subsumes several functions. It's big. It's not small like init. It's not fork/exec simple it forks control processes and monitors children and does a lot of work some daemons did themselves (getty, inetd) but others didn't do. And you get to monitor things. It's not unlike launchctl on OSX or other platforms. And after all /etc/rc is now 5 or more/etc/rcX.d/ complex substates of states to look for scripts, SAT solver complex dependency stuff between semi arcane shellscripts... blah.

PRO: everything is now much more consistent

CON: greybeards like me hate it (beardless btw but meh) because it's complex