In a physical console (like the Xbox 360), the BIOS is a small chip on the motherboard that contains low-level software. When you turn on the console, the BIOS is the first code that runs. It initializes the hardware (GPU, CPU, RAM), checks for connected controllers, and then hands control over to the operating system (the Xbox 360 Dashboard).
: Xenia supports common Xbox 360 file formats, including .iso disc images and .xex executable files. xenia bios files