Challenge 1: Adding a Mask While a Selection Is Active — Solution Walkthrough

What happens:

Per this chapter's own material, when a mask is added while a
selection is still active, that selection automatically becomes the
mask's own black-and-white pattern. The selected area (the person)
becomes visible — white on the mask — and everything outside that
selection becomes hidden — black on the mask — all in one single
step, with no manual painting required at all.

Why this works exactly as described:

This is the direct bridge between Chapter 3's own selection tools and
this chapter's own masking concept: the careful, precise selection
already built with the Lasso/Free Select tool doesn't need to be
manually repainted onto a mask by hand — the selection itself becomes
the mask automatically the moment the mask is created.

The practical result:

Only the selected person remains visible on this layer; everything
outside the original selection is hidden, but per this chapter's own
core distinction, none of the actual pixel data outside the selection
has been deleted — it's simply hidden and could be restored later by
painting white over that area of the mask.

WHY THIS WORKS AS AN ANSWER
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This exercise is a direct, hands-on application of this chapter's own
selection-to-mask material, correctly connecting Chapter 3's own
selection work to this chapter's own mask-creation process.