Once you have the legally dumped BIOS files (or decided to risk downloading them, which we strongly advise against), here is exactly how to install them for Citra on Android.
Downloading BIOS or system files from third-party websites is often a legal gray area or contains malware. The recommended and safest method is to : 3ds bios file download for android
Use a modded 3DS with GodMode9 to extract necessary files like aes_keys.txt or seeddb.bin . Once you have the legally dumped BIOS files
try (Response response = client.newCall(request).execute()) if (!response.isSuccessful()) throw new IOException("Unexpected code " + response); which we strongly advise against)
Citra is an open-source emulator that supports many 3DS games.