12.17This is the orange ring in your chart

Now the chart should make much more sense.

If the outer ring has:

C → G → D → A → E...

then dominant sevenths point toward those destinations:

G7 → C

D7 → G

A7 → D

E7 → A

and so on.

The orange chords are essentially a network of:

“Here is the dominant that points to that chord.”