It's dependent on the manufacturer to decide if they want a black box system management processor, what architecture to use, and what it will be responsible for.
The Raspberry Pi SoC uses the video accelerator processor (VPU) for bootstrapping the ARM CPUs - effectively making the ARM a coprocessor to the VPU. The config.txt and *.elf files you have to put on the SD card are the OS for the VPU and bootloader for Linux that gets loaded to the ARM cores.