Web: https://systemd.io/
Src: https://github.com/systemd/systemd
Systemd manpage index: https://www.freedesktop.org/software/systemd/man/
https://www.freedesktop.org/software/systemd/man/systemd.htm... :
man 1 systemd
man systemd
man init
...: man systemctl
man journalctl
man systemd.timer
man systemd-resolved
The Arch Linux wiki docs for systemd are succinct:
https://wiki.archlinux.org/title/systemdFedora docs > "Understanding and administering systemd" https://docs.fedoraproject.org/en-US/quick-docs/understandin...
RHEL7 System Administrator’s Guide > "Chapter 10. Managing Services with SystemD" https://access.redhat.com/documentation/en-us/red_hat_enterp...