An explanation of systemd for the beginner, with a tutorial configuring a simple service.
References:

https://gist.github.com/funzoneq/737c...
https://wiki.debian.org/systemd
https://wiki.debian.org/systemd/Services
https://www.linux.com/training-tutori...
https://opensource.com/article/20/5/s...
https://www.loggly.com/ultimate-guide...
https://firewalld.org/documentation/h...
https://www.digitalocean.com/communit...
https://www.digitalocean.com/communit...