The C major scale is played by going from C to C on the white keys of the piano like this.
The A minor scale is played by going from A to A on the white keys.
Because these 2 scales have the exact same notes in them (i.e., they both use only white keys) they are called “relatives.”
A minor is the relative minor of C major
C major is the relative major of A minor
So the only difference between the two is what you are calling your ROOT note, which is your HOME BASE, the place of greatest rest and resolution.
To find the Relative Minor of a major key you can count up 6 notes of the scale, or go down 3.
So A minor is the relative minor of C major
Or, maybe it’s easier to count the other way?
So A is the natural minor of C major.
The difference between the tonality of major and minor lies in the order of the half-steps, which creates different intervals.
If you look at the notes of the C major scale, the order of whole-steps and half-steps is:
So the half-steps are between 3 and 4, and 7 and 8.
In the A minor scale, the order of whole-steps and half-steps is:
So the half-steps in the minor scale are between 2&3 and between 5&6, (and because of the different order, the 3rd, 6th, and 7th degrees of the minor scale are a half-step lower than that of the major scale.)
But still, A minor and C major share all the same notes, and that’s why they are called relatives.