On the piano
Every coloured key is a note of A natural minor, and every one is also a note of C major. The seven keys are the same, and only the note treated as home has changed.
A natural minor scale on the whole neck
84 notes of the A natural minor scale are marked across six strings.
The neck shows the same thing, because every dot here is also a note of C major. Play the shape starting from C and it sounds major. Start it from A and it sounds minor.
A minor and C major contain exactly the same seven notes. If you play the white keys from C, the result sounds major. If you play them from A, it sounds minor. The only thing that has changed is which note you treat as home.
Every major key has a minor partner like this, called its relative minor. It starts on the sixth degree of the major scale, three semitones below the major root. The two share a key signature, so on paper you cannot tell which one a piece is in. You can tell them apart by which chord the music keeps returning to.
For a guitarist this saves real work. The shape you learned for C major is already the shape for A minor, in exactly the same place on the neck.