Exercise 2: webapp.service in Both /usr/lib and /etc — Which One Wins — Possible Solution
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Answer: systemd uses the ExecStart= value from the copy in
/etc/systemd/system/, not the one in /usr/lib/systemd/system/.

Explanation:

Per the chapter's own location-precedence material, /etc/systemd/
system/ is explicitly described as holding "local admin overrides and
custom units, taking priority over the vendor copy" in
/usr/lib/systemd/system/ (or /lib/systemd/system/). When systemd
encounters a unit with the same name present in more than one of its
own search locations, it doesn't merge or average the two -- the
chapter is explicit that "/etc wins over /usr/lib." This is exactly
the mechanism the chapter names as intentional: it lets a local
administrator override a package-provided unit's behavior (in this
case, whatever webapp.service's own ExecStart= actually launches)
without editing the original package-shipped file directly -- which,
per this chapter's own warn-box, would risk being silently
overwritten by a future package update targeting
/usr/lib/systemd/system/. The /etc/systemd/system/ copy is genuinely
the one systemd will act on when the unit is started, restarted, or
reloaded.

WHY THIS WORKS AS AN ANSWER
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This states the correct answer (/etc wins) and justifies it by
directly quoting the chapter's own stated precedence rule and its
underlying purpose (safe local overrides that survive package
updates), rather than just asserting which file "should" apply.