Check file permissions. On Android 11+, RetroArch cannot read the system folder if you moved it via USB. Use RetroArch's built-in file browser to "Copy" the BIOS from your Downloads folder to the system folder within the app.
psxonpsp660.bin BIOS is widely considered the "best" or "gold standard" BIOS for PlayStation 1 (PSX) emulation in Psxonpsp660.bin Retroarch BETTER