For years, the BIOS was an absolute requirement. If you didn't have the .bin file inside that .zip , your screen stayed black.
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. PS2_BIOS.zip
To play a PS2 game on a computer or phone, you need an emulator (like PCSX2 ). However, the emulator is just a shell. It provides the "muscles" (hardware simulation), but it lacks the "brain" (the BIOS). For years, the BIOS was an absolute requirement