The Atari Jaguar is emulated by the Libretro Virtual Jaguar core. Your games go in /recalbox/share/roms/jaguar and use the .j64, .jag, .rom, .bin and .prg extensions, optionally inside .zip or .7z.
No BIOS is needed for most games. However, some titles refuse to boot until BIOS emulation is enabled in the core options. That is the main sticking point on this system.
HOTKEY + B.Options (core options).Bios option to enabled.Back, then Resume, and quit the emulator entirely.The setting is kept for the whole system.
📷 Screenshot coming:
jaguar-bios-option.png
If you would rather stay out of the menus, or only enable the BIOS for certain games, use a configuration override.
Create a file named .core.cfg containing:
virtualjaguar_bios=enabled
Put it in /recalbox/share/roms/jaguar to cover the whole system, or in a subfolder to cover only the games it contains.
Leaving the BIOS permanently enabled adds a little to game boot times, but causes no compatibility issue: if you would rather not think about it again, just turn it on for good.