An XCI file contains the contents of a Nintendo Switch game card in the NX Card Image format. It stores an encrypted backup of a Nintendo Switch game, which includes the Switch game ROM, icons, and metadata. XCI files may also be used to contain updates to a Switch game.
XCI files can be played on a Nintendo Switch using SX OS, which can be booted on to a Switch console using the Xecuter SX Pro tool. The SX OS is custom firmware that allows you to play Nintendo Switch Homebrew and XCI backup files. XCI files require master keys to decrypt them so they should only be used if you own the game.
XCI files may also be played using Switch emulator, such as Yuzu or Ryujinx, but they must be decrypted to .NRO files. You can decrypt XCI files using hactool, which is a utility developed by SciresM to decrypt, extract, and view information about various Nintendo Switch file formats.