Exercise 1: start vs. enable, and What Happens at the Next Reboot — Possible Solution
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"systemctl start nginx" on a currently-stopped, disabled nginx:

This immediately starts nginx running right now, per the chapter's own
description of start/stop/restart as controlling "whether the unit is
running right now." Nothing about this changes nginx's enabled/disabled
status at all -- it remains disabled. Per the chapter's own compare
table, this action does NOT survive a reboot on its own -- if the
system reboots, nginx will be stopped again, since nothing was ever
configured to start it automatically at boot.

"systemctl enable nginx" on the same system:

This does the opposite kind of thing -- per the chapter's own
description, it creates the symlink tied to nginx's own [Install]/
WantedBy= line, configuring nginx to start automatically at the next
boot. Critically, enabling does NOT start nginx immediately -- if
nginx was stopped before this command, it remains stopped right after
running it too. Nothing changes about its CURRENT running state.

-- What happens at the next reboot in each case --
--
-- After only "systemctl start nginx" (with no enable): at the next
-- reboot, nginx will NOT be running, since starting alone never
-- configured it to start automatically -- the deliberate one-time
-- start doesn't persist across a reboot.
--
-- After only "systemctl enable nginx" (with no start): at the next
-- reboot, nginx WILL be running, since the boot-time symlink is now
-- in place and systemd will start it as part of normal boot -- even
-- though it wasn't running in the moment right after the enable
-- command itself.

WHY THIS WORKS AS AN ANSWER
------------------------------
This works through both commands separately, explicitly addressing
both the immediate effect and the reboot-time effect for each one
(four total states: current state after each command, and post-reboot
state after each command), rather than conflating the two axes the
chapter's own compare-table keeps separate.