Are there any 3rd party repos for NixOS? It seems some people overlay one package, but I cannot find entire packagesets such as the one in Nixpkgs. Are channels used for that? Would I be able to have my own repo with several packages? Can I have more than 1 repo in my Nix system, and set which package install each program from?

Yes, the most common one I think is the NUR, the Nix User Repository, the packages have their own namespace. https://github.com/nix-community/NUR

Would it be possible, for propietary software compiled directly for NixOS, to have a binary-only (as opposed to source, or binary+patched) repository? Everything I see is either source+cache or binary+patched, but nothing like binary especifically for NixOS.

I don’t see anything prohibiting that

Let’s say I want to add some extra, propietary codecs unavailable to NixOS. How would I be able to change a dependency so that each package dependent on it automatically picked it? Could I switch a dependency for a certain repo from one of another, different repo?

The issue is that packages (or derivations) only depend on something because it’s declared in their definition. That means if you need to add dependencies to existing packages, that can be done via overrideAttrs. However, I’m not certain that you can easily add attributes to functions that don’t have a mechanism for it (I don’t know derivations that allow for more attributes than specified and I’m not very good at the nix language) and dependencies (buildInputs and nativeBuildInputs) can only use packages that were supplied as attributes. Derivations usually declare their attributes via { x, y ? "bar" }:, what you would need to make use of additional attributes is something like attrs @ { x, y ? "bar", ... }: so that the additionally supplied attributes can be accessed. But again am not an expert in the language so take that with a grain of salt.

Are there any 3rd party repos?

Nix has “flakes”, which allow you to share Nix code in a Git repository, it’s like repos on steroids. There are many Git projects that offer new packages (such as nix-gaming) or NixOS modules (such as my project nixos-router), or even just Nix code (such as my projects notlua and notnft, which allow you to write Lua code and nftables rules in Nix), or any combination thereof.

Would it be possible [for proprietary software to be compiled for NixOS]?

Kind of. You first have to understand what Nix derivations are - builders that take certain inputs (such as certain versions of libraries) and produce some outputs.

What happens if the inputs (such as a library version) change? The outputs change as well - previously it was /nix/store/abcdefgh-libfoo/lib/libfoo.so, now it’s /nix/store/ijklmnop-libfoo/lib/libfoo.so - the path to libfoo changes, and the binary’s RPATH reflects that.

So if you want to package binary software for NixOS, you either have to pin library versions (so the paths don’t ever change), or patch the binary.

…proprietary codecs…

It depends on what those codecs are.

Let’s say they are a binary. In that case, you install them and they get added to your PATH - easy.

Let’s say they are some data files. In that case you install it and it gets put into XDG_DATA_DIRS - easy.

Let’s say it’s a shared library (.so). First question - how is that .so loaded? By which program? From where?

Depending on the answer, what you have to do changes as well. You may have to override some core media library, or ffmpeg, or maybe you can override VLC, or VLC’s ffmpeg, but not system ffmpeg. Or, it may be the case that a simple LD_LIBRARY_PATH change will do it for you.

Basically - it depends. That’s why NixOS requires a deeper knowledge of Linux, or forces you to learn.