3.10Very important: minor keys routinely borrow this raised 7

If a piece is in A minor, you may see:

G natural

in one passage,

and:

G♯

in another.

That does not necessarily mean the piece changed key.

The raised seventh is often introduced specifically to strengthen:

V → i

So minor tonality is more flexible than a beginner's idea of "one key = one fixed set of seven notes."