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I'm new to NixOS. Do I have to do anything extra to update NixOS? Or do I just update my flake and run nixos-rebuild switch --flake like I normally do to update packages?
If you are using flakes you should check your flakes' inputs (probably the one called nixpkgs) and then change the URL to match the channel for 23.11.
Finally, you should of course rebuild your system.
If using flakes you could just for instance add another input. You can also set the input URLs to specific states of the nixpkgs repository by eg referencing specific commits. Then, you should be able to just, e.g., pick Firefox from unstable, another package from the current stable channel, and maybe a broken package from a pull request fixing said package.
If you are not using flakes you can also add system wide channels. IIRC you can then import these channels into your configuration.nix and select packages from the corresponding channels. But here the channels/inputs are not part of configuration itself in contrast to when using flakes.
I'm a bit confused about what you actually want? Do you just want to update your packages, but stay on the same NixOS version? Just continue like before. Do you want to stay on your current version, but use some packages from the next version? That should also be possible if you somehow include that channel in your configuration.nix (though I don't know how this would work in practice).
Personally, I just run with unstable though, then the releases aren't that important.
I think unstable and the fixed versions use the same Firefox package, so you wouldn't gain anything. The difference is rather in libraries that get used and how the distribution does things. For example, the changes listed in https://nixos.org/manual/nixos/stable/release-notes#sec-release-23.11-incompatibilities just appeared mostly one by one for me; one day, I wanted to update my system and got the error that the fonts option got renamed, so I had to change my configuration.
The fonts.fonts and fonts.enableDefaultFonts options have been renamed to fonts.packages and fonts.enableDefaultPackages respectively.
While when using a fixed point release, these changes won't happen. Only when you switch releases. That's what "unstable" refers to.
When not using flakes, nixos-rebuild switch --upgrade is equivalent to apt update; apt upgrade. The equivalent to dist-upgrade is nix-channel add $NEW-CHANNEL-URL nixos and then performing a regular update.
You need to update your inputs so that you're using the 23.11 branch of nixpkgs instead of the old one. In my experience, a couple of things will break, but there's usually warnings about it.