GB, the making of. I'm not a musician, how this Apps was made in term of chords.

Question

Hello from sunny Geneva,
I really love this application and spend hours playing with.
It's a huge question maybe, but I'd like to understand how
this amazing Apps was made in term of choice for the chords,
for those smart instruments. Piano, Guitar, Bass, Strings
Why Em Am Dm G C F Bb Bdim
Why presented/listed in this order Em to Bdim
About guitar : for a chord, how was made the choice
for the 6 notes composing the chord
i.e. from bottom G C E G C E for C chord
What chords are behind when I press 2 or 3 fingers
to change the chord ?
The same question for bass guitar, piano, strings...
I just search for the making of as I know how to use it,
creating music, exporting to Mac or YouTube.
I try to undrestand using Google or through books
but it is very difficult.
Thanks in advance for your posts.
Franck

Answer 1

iii-vi-ii-V-I-VI-VII-vii is a major circle progression with a borrowed VII from the minor scale, and this is the order of the chords in the key of C
if you change the Key of the song in garageband, the chords will change but follow this same pattern
the vi–ii–V–I progression is a chord progression (also called the circle progression for the circle of fifths, along which it travels). It is "undoubtedly the most common and the strongest of all harmonic progressions" and consists of "adjacent roots in ascending fourth or descending fifth relationship"
- from circle progression wikipedia article
music theory is designed so notes and chords sound good, and garageband is just following the formula to make it fun and easy!