According to copyright law, the only 100% legal way to obtain the BIOS is to "dump" it from a physical PS2 console you own using specialized software.
For those without the technical skills or hardware, PS2_BIOS.zip is found in the shadows of the internet, hosted on sites like The Internet Archive or shared in Reddit Megathreads . PS2_BIOS.zip
What Is the PS2 BIOS? How It Works and Why Emulators Need It According to copyright law, the only 100% legal
The BIOS (Basic Input/Output System) is the proprietary code Sony wrote to start the console and tell it how to read a disc. How It Works and Why Emulators Need It
The story of this file is defined by a strict legal boundary that most users quietly ignore.
Because Sony owns the copyright to this code, emulator developers cannot include it in their software without facing massive legal consequences.