Do you currently have installed on your physical 3DS?
The safest way to obtain a BIOS file for your 3DS is directly from Nintendo or through official channels. However, if you're looking to download a custom BIOS or firmware, make sure to research and find a trustworthy source. Nintendo 3ds Bios File Download
provides a step-by-step walkthrough on which specific folders you need to copy for proper emulation. Nintendo's Stance Nintendo's IP Policy Do you currently have installed on your physical 3DS