Challenge 1: Loaders vs. Plugins — What Each Operates On — Possible Solution
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WHAT A LOADER OPERATES ON
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Per this chapter's own css-loader example, a loader operates on
INDIVIDUAL FILES as Webpack's own dependency graph processes them.
css-loader specifically transforms a single .css file's contents into
a form a JavaScript module can consume, which is what makes writing
import './styles.css' inside a JS file possible at all — the browser
itself has no concept of importing a stylesheet like that; css-loader
is what bridges the gap for that one file, every time Webpack
encounters a file matching its configured pattern.

WHAT A PLUGIN OPERATES ON
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Per this chapter's own HtmlWebpackPlugin example, a plugin operates
on the BROADER BUILD PROCESS itself, not any single file.
HtmlWebpackPlugin doesn't transform any particular source file at
all — instead, it hooks into the overall build (specifically, after
the bundle has been generated) to automatically produce a new HTML
file that references the finished output. It's working at the level
of "the whole build just finished, now do something with the
result," not "this one file needs to be converted into something
else."

THE CORE DISTINCTION
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A loader's scope is per-file: it runs once for each file matching its
configured test pattern, transforming that file's own content. A
plugin's scope is the entire build: it can hook in at various stages
of the whole process (before bundling starts, after it finishes,
etc.) and act on the build as a whole rather than on any specific
file's contents.

WHY THIS WORKS AS AN ANSWER
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It identifies precisely what each mechanism acts on using the
chapter's own two named examples (css-loader for per-file
transformation, HtmlWebpackPlugin for whole-build-level action), and
states the core scope distinction (per-file vs. whole-process)
directly rather than describing the two as simply "different kinds of
plugins."