Some emulators need BIOS files to work. Those files can be downloaded from here

NES emulator - Mesen

Sega Master System / Game Gear emulator - Snepulator

Game Boy / Game Boy Color emulator - SameBoy

Sega Genesis / Mega Drive emulator - BlastEm

SNES, N64 emulator - ares

Sega Saturn emulator - Mednafen

PlayStation (one) emulator - DuckStation

Sega Dreamcast emulator - Flycast

Playstation 2 emulator - PCSX2

Xbox emulator - xemu

Game Boy Advance emulator - mGBA

Playstation Portable emulator - PPSSPP

Playstation 3 emulator - RPCS3

Gamecube, Wii emulator - Dolphin

Nintendo DS emulator - melonDS

Wii U emulator - Cemu

Nintendo 3DS emulator - Citra

Playstation Vita emulator - Vita3K

Nintendo Switch emulator - Ryujinx