You must dump games from your own physical cartridges using a homebrewed 3DS or purchase digital copies. Downloading ROMs from the internet is copyright infringement.
For most modern 3DS emulators on Android, a traditional (like those needed for PlayStation or GameCube) is generally not required to play games. Instead, 3DS emulators often require (stored in an aes_keys.txt System Archive 3ds Emulator Bios File Download For Android
For the Nintendo 3DS, the BIOS (often referred to as "boot ROM" or "firmware") is responsible for: You must dump games from your own physical
– Nintendo 3DS BIOS files are proprietary, copyrighted system firmware. Downloading them from unauthorized sources (i.e., anywhere other than dumping them directly from your own personal, legally obtained Nintendo 3DS console) is copyright infringement and violates DMCA and similar laws worldwide. 3ds Emulator Bios File Download For Android