The documentation states that the default cache dir on an XDG compatible *nix system is $XDG_CONFIG_HOME/composer/cache but composer uses XDG_CACHE_HOME/composer as default.

https://getcomposer.org/doc/03-cli.md#composer-cache-dir quote:

By default it points to $COMPOSER_HOME/cache on *nix and macOS, and C:\Users<user>\AppData\Local\Composer (or %LOCALAPPDATA%/Composer) on Windows.

• On an XDG compatible *nix system the default $COMPOSER_HOME is $XDG_CONFIG_HOME/composer
  • Therefore $COMPOSER_CACHE_DIR -> $COMPOSER_HOME/cache -> $XDG_CONFIG_HOME/composer/cache
• composer uses $XDG_CACHE_HOME/cache instead as cache: https://github.com/composer/composer/blob/master/src/Composer/Factory.php#L116