Just so it's clear for everybody: Nix is a programming language, build system, and package manager. NixOS is a Linux distro built with (and upon) Nix. Home Manager is a dotfile and home management tool using Nix, allowing control of dotfiles, but also per-user software, systemd services, and more. You can use Home Manager in any distro, not just NixOS (but you do need to install Nix).

Here's my example (Github mirror). It stores everything from my custom packages (like GIMP 2.99, which isn't yet packaged in nixpkgs, or a custom virtiofsd to workaround an upstream bug caused by switching from the old C to the new Rust implementation), to my fish, sway, rofi, mpv configs, to my entire server setup, including Gitea, Nextcloud, Keycloak, Mumble, mailserver and Matrix server with some bots and bridges (I recently migrated from an x86_64 to a arm64 board and the only post-install setup I had to do was copy /var), to my router's nftables rules.

I just use Git for my nixos config files.