It all goes back to Pythagoras, the old Greek who is the bane of high school geometry classes. Without going into detail about how he went about it (experimenting with harmonics on a stretched string and tuning other strings to those harmonics), he came up with the modern 12-tone chromatic scale. That is, twelve distinct notes before he started repeating himself.

If you use all twelve notes in a piece of music, you have no center, or "home plate." So various people "selected" notes from the available twelve, and the modes developed.

This is really an overly simplistic explanation, but in the interests of brevity. . . .

The major scale is actually the Ionian mode, which, along with other modes, had been used for centuries. The natural minor scale is the old Aeolian mode. Same notes as the Ionian mode, but starting and ending at a different point, hence the "relative minor." These two modes, used separately or in combination, proved to be the most musically versatile, so they are the ones used primarily in classical and popular music from, say, the Renaissance on (give or take a century or two).

It's hard to make these things clear without writing a book—and, indeed, there have been many books written on the subject.

The "seventh chord," as in "dominant 7th," of, say, the key of C, is G7, where the 7th in question is the interval between the G (root of the chord) and the F natural. F natural rather than F#, because we're dealing with the key of C, not the key of G.

As to the use of major 7th chords (or 6ths, 9ths, and other notes added to the basic triad), these are "color chords" and are never actually necessary, beyond stylistic conventions.

Don Firth