RetroArch is an all-in-one frontend that requires precise file placement. Locate your main installation folder. Open the system folder. Create a new folder inside it and name it lowercase dc .
Legally, BIOS files are copyrighted software owned by Sega. Sharing or downloading these files from public internet repositories violates copyright laws. The strictly legal method to acquire a Dreamcast BIOS is to dump the firmware directly from your own physical Sega Dreamcast console using homebrew software and an SD card adapter. Finding Files Online Safely bios sega dreamcast