Similar to RetroArch, it looks for a "data" or "system" folder to pull the boot sequence. A Note on Legalities and Sourcing
If you are installing an Optical Drive Emulator (ODE), having a backup of the BIOS files allows you to verify that your hardware is communicating correctly with your SD card or SSD storage. Common Files Found in a Dreamcast BIOS Pack dreamcast bios pack
If you're interested in downloading a Dreamcast BIOS pack, be sure to check out reputable sources, such as: Similar to RetroArch, it looks for a "data"