I was also confused about why my images contained the ZFS kernel module but my config didn’t.
Lists are merged together by the module system.
You set: boot.supportedFilesystems = [ "btrfs" "reiserfs" "vfat" "f2fs" "xfs" "ntfs" "cifs" ];
But because you’re also importing nixos/modules/installer/cd-dvd/installation-cd-graphical-calamares.nix that imports multiple files. You’ll eventually find nixos/modules/profiles/base.nix which also sets boot.supportedFilesystems.
I can highly recommend using nix repl or tools like nix-inspect to inspect the config attrset and see the “actual” values.