The BIOS determines where the console can play games (e.g., SCPH-1001 for North America, SCPH-1002 for Europe).
The infamous Sony Computer Entertainment logo, the sound, and the final "diamond" splash screen are all rendered by the BIOS before a game loads.
If you are setting up emulators (like PCSX2, which also runs PS1 games), you will likely need one of these ROM files: North America (US) SCPH-1002: Europe (EU) SCPH-1000: Japan (JP)
The (Basic Input/Output System) is the essential firmware—the "soul"—that runs the console. It initiates the hardware, manages the iconic startup animation, and handles the region locking that defined the 32-bit era.