You are confusing scales with key elfascinating.
All songs in popular music are set to a key (major or minor) from where the chords are derived.
Chords of a major key are
I IIm IIIm IV V VIm VIIdim
The m represents a minor chord and not the interval.
Chords of a minor key are
Im IIdim III IVm Vm VI VII
And as you may already know the changed intervals are minor III, minor VI and minor VII (the chords are major though)
Which means that in the key of G major the various chords you can choose from are
G Am Bm C D Em F#dim
While in the key of G minor the chords would be
Gm Adim Bb Cm Dm Eb F
So any song which includes only these chords would completely be in the key of G major or minor depending on the chords you choose.
Musicians create variety by choosing chords that seem to be out of key at first, but if you analyse the progressions you may find a logic in the way they are progressing. This is especially notable in jazz music.
Scales are one way of playing the melodies over the given chords. They represent notes whose intervals with the chord tones are consonant or even dissonant, and the musician is free to choose which notes he wants, to generate whatever feeling/mood he is trying to achieve.
It is important to know that while arpeggiating chords is usually the most consonant way to follow changes in a song, many different scales can contain the same chord tones, but the other notes in the scale may provide the musician with many more notes to put up against the chords.
Even though a lot of the musical greats of old (famous or not) didn't have a formal music theory education, they knew exactly what they were doing. They knew their fretboards inside out, and in their mind, from experience and from their own creativity, they knew what notes/licks the wanted to play, and when. Sure they didn't know that the scale had a name, but they knew the set of notes they were choosing from. Sure they may not have known beforehand which intervals are consonant and which are dissonant but they knew exactly which notes sounded good against a certain chord.
Music theory gives you a more technical path to the same goal - to be able to make beautiful music.
Everything's Eventual (tm)
Scarborough Fair/Canticle (Cover)