Overlays for both NixOS and Nix-Darwin.
If you don't know much about overlays, it is recommended to learn the function and usage of overlays through Overlays - NixOS & Flakes Book.
default.nix: the entrypoint of overlays, it execute and import all overlay files in the current directory with the given args.fcitx5: fcitx5's overlay, add my customized Chinese input method - 小鹤音形输入法