When you press the power button, the CPU doesn't start at the BIOS. It starts at a specific memory address that "aliases" to the secret MCPX ROM.
Not all Xbox consoles are identical, and the MCPX evolved over time. Mcpx Boot Rom Image
If you are a modern Xbox modder, you might be asking: "I have an OpenXenium modchip. Why do I need to know about the Boot ROM?" When you press the power button, the CPU