These files are used to install games, updates, or DLC directly to the 3DS home menu.
To use a CIA file, a 3DS must usually have Custom Firmware (CFW) like Luma3DS installed.
Since March 2023, the 3DS eShop no longer allows new purchases. This has led many to "abandonware" sites for game preservation.
If you don't have a 3DS, you can run these files on a PC using the Citra emulator (or its forks). 🛠️ How to Use a CIA File
Navigate to the file and select "Install and delete CIA."
Open the "FBI" homebrew application on your 3DS.