Challenge 1: Placing a Transparent-Background Character in Front of a Rendered Background — Solution Walkthrough

The answer:

Per this chapter's own material, an Alpha Over node accomplishes this
directly. It specifically layers one image on top of another using
its own transparency information, placing a foreground element
convincingly in front of a separately rendered background — exactly
what's needed to combine the character's own transparent-background
render with the separately rendered background environment.

Why a plain Mix node wouldn't be the right choice here:

Per this chapter's own material, a Mix node blends two image inputs
together according to a chosen blend mode, without specifically
respecting one image's own transparency information the way Alpha
Over does. Since the character was rendered with a transparent
background specifically so it could be placed over something else,
Alpha Over is the node built to actually use that transparency
correctly.

Why this workflow makes sense in the first place:

Rendering the character and background separately, then combining
them with Alpha Over, is exactly the kind of "separately rendered
elements combined after the fact" workflow this chapter opens with —
splitting the heavy rendering work from the more flexible work of
combining the pieces.

WHY THIS WORKS AS AN ANSWER
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This exercise checks that combining a transparent-background render
with a separate background is correctly matched to the Alpha Over
node, rather than a generic Mix node.