Painless wire harness install (MORE PROGRESS)

I can't know how the fusebox is made inside or why. Most have several seperate buss bars with their seperate power supplies even though more than 1 of those power supplies originates at the same source. The why can be related to wire gauge just as others have suggested.
I'm fairly good at figuring out the OEM methods to the madness but honestly..
Idont know squat about aftermarket wiring.