Hello everyone
This wonderful book has just appeared in a new 3rd edition.
Remember, if you buy it directly from the publisher, "No Starch Press", and choose to buy the paper edition, you get a drm-free ebook copy (formats epub, pdf, mobi) gratis .
Link: https://nostarch.com/howlinuxworks3
Chapter 6, section 6.3 here I come... (systemd)