Few links I found, for anyone else encountering this for the first time:
https://rasmuskirk.com/articles/2024-07-24_dont-use-nixos/
https://tech.aufomm.com/my-nix-journey-use-nix-with-ubuntu/
https://jeancharles.quillet.org/posts/2023-02-07-The-home-ma...
https://www.reddit.com/r/NixOS/comments/16tdugd/nix_without_...
https://www.reddit.com/r/NixOS/comments/1lbv8hn/ive_avoided_...
- nix primarily is the package manager; basically can be installed anywhere. The big thing is; it builds packages to /nix/ , keeps software there and all the dependencies are done through symlinks to packages in the nix store - nix is also the functional programming language that the nix package manager can take as input - nixos is a framework for writing a system configuration in the nix language; and produces a working linux distribution; integrated with nix. - home-manager is copying the nixos framework, but brings it to userspace management.
The biggest issue getting into home-manager from outside is; it's high learning curve and due to being in the nix ecosystem; there is a lot of concepts that it uses (modules, configuration in functional paradigm) that is a whole lot to learn from 0 just to manage system config, but once things click together, it is wonderful.
To add to what you have found; there is also https://github.com/numtide/system-manager which brings NixOS' declarative system configuration to other distros as well, but I haven't personally used it.
Technically you could use plasma-manager on any distro; you install nix, and use home-manager standalone. I think with KDE; you would want to skip package management, but doable; I used to do with arch.
I guess depends on what you already use for configuration management. Bear in mind; a lot of KDE apps do store state information (window size etc.) inside the config directory and there wasn't an easy way to disentangle config from state when I used to manage my config with a git repo.