How does the fuse box get power?

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Ive looked over the wiring for months now, i have several books with diagrams, seems the entire box is powered by the 12red splice?

New fuse boxes have a main hot bar like in a houses circut breker box,but these boxes tend to just all be an inline fuse holder basically...

correct me if im wrong, but is everything powered by the 12red splice??? If so thats a lot of current for a 12g.

UPDATE: yes i know this 12red snakes to ign switch in 2 spots and the 1 to "horn relay" and other to headlight sw.

Im just confused please someone explain, when i disconnect this circuit off firewall block i will loose power to entire truck.

EXAMPLE: how does blower motor get power, dome, brake??? this has to be a main branch in the box somewhere to where all spider web off other circuit..... i dont know now im just rambling...