A rudimentary question is a wonderful an opportunity to distill everything down to a simple understandable concept.
Here's how I would do it:
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Western music is based on simple frequency ratios. 2:1 is so basic that we call it the same note, but an octave up.
Three pitches with ratios 4:5:6 make a major chord. These are the simplest possible ratios within an octave, so a major chord is the most basic chord, and all other chords are derived from that