You cannot install CIA files on a "stock" (unmodified) 3DS. If you haven't modded your system yet, the gold standard guide is 3DS.hacks.guide .
A file is a package format used to install games, updates, or DLC onto a 3DS console using custom firmware (CFW). Stretchmo 3DS (USA) DLC CIA
Finding specific "CIA" files for Nintendo 3DS games like can be a bit tricky because the official Nintendo eShop for the 3DS has been closed. Since Stretchmo is a "free-to-start" game, the actual puzzles and levels were sold as Downloadable Content (DLC), which are now difficult to acquire legitimately. What is Stretchmo? You cannot install CIA files on a "stock" (unmodified) 3DS
Since sharing direct download links to copyrighted CIA files is often restricted on major platforms, the 3DS community relies on a few well-known archives: Finding specific "CIA" files for Nintendo 3DS games
