When you type "Citra Aes Keys.txt Download" into Google, you are looking for a pre-made file. While you can find these files on various ROM-hosting forums and GitHub repos (often named aes_keys.txt or citra_aes_keys.txt ), doing so carries significant risks:

Many users prefer to decrypt their game files ( .3ds or .cia ) directly on their 3DS before moving them to a PC. Decrypted games do not require an aes_keys.txt file to run in Citra.

The Nintendo 3DS uses to protect its software from piracy and unauthorized use. Every game (ROM) you dump from your console is typically encrypted.