The file is a dump of the PS1 BIOS extracted directly from the PSP’s 6.60 firmware. Why use this version specifically?
Place the file in the system folder of your RetroArch directory. psxonpsp660.bin github
Once you’ve located the file on GitHub, here is how you typically use it: For PPSSPP (PC/Android) Download the psxonpsp660.bin file. The file is a dump of the PS1