In the context of the Sega CD (known as Mega CD outside North America), these .bin files are exact digital copies of the physical ROM chips located inside the Sega CD add-on hardware. When you turn on a physical Sega CD unit, you see the distinctive "logo animation" and the "Player" screen where you can listen to music CDs. That software is the BIOS.
for these files in a particular emulator like RetroArch or Lemuroid?
The BIOS is firmware that is embedded in a computer's motherboard. It serves as the interface between the computer's hardware and its operating system. The BIOS is responsible for initializing and testing the system's hardware components and loading the operating system.