I've been a Debian guy for many years and it's unlikely I'll move away in the foreseeable future.
Nix is a package manager and nix-os is an OS built around that package management system.
You can install nix the package manager in debian. I don't use it for installing desktop apps like a browser or office suite, I prefer AppImages for those. However, it's absolutely fantastic for CLI stuff, especially the things you might want as a once off. You can just nix-shell -p <obscure cli tool> and it's just magically there in a new temporary shell, and then cleaned up once you quit that shell. No more adding weird repos to apt, or downloading from github and building, or piping scripts to bash.
There's also home-manager which allows you to define packages and their configurations, and just roll out that state on any machine.
These fancy package management tools (flatpak, AppImage, and nix) have dramatically changed the Debian experience. I used to be forever struggling with the trade off between stability and old versions of things. That's really not the case any more because you don't have to interfere with Debian's conservative methodical ideology around stability in order to install and use all the shiny new things.