Aethersx2 (now often continued as ) requires a PlayStation 2 BIOS file to function. While BIOS files come in different sizes, the 4MB version is the standard "v2.0" or later dump from Slim model consoles, which is generally considered the most compatible.
Finding the BIOS is the user's responsibility, but setting it up within the emulator is a straightforward process. Here is how the system works once you have the files. aethersx2 bios 4mb