What Is Nix?

General installs from Nixpkgs aren't going to be highly-idiomatic. You can install two different versions of coreutils, but one of them is still always going to be first on your PATH. So, if you want to specify the version in each command like this, you'll have to override the expressions to write a version number into the name of each binary. (But, yes, they're both on your PATH. If by "easily" you mean you could run `type -a` and pick the correct full path for each and then run them in a pipeline like this--yes.)

I guess you could also build the path of each into an environment variable, or maybe create a wrapper for each with the version name included.

Second: This is an awkward example because people rarely use multiple versions of coreutils concurrently, so there's just one version at a time in a current version of Nixpkgs.

It'd be easy to put more than one python or ruby or postgresql in the same expression--things people are more likely to need (and thus multiple recent versions are all available at once)--but you'll (probably) have to work a little harder to do something like coreutils. (i.e., copy in a complete expression for each, or pull each version from two different commits in Nixpkgs.

Nix makes it fairly simple to override an existing expression to change something like the version/commit used, but it breaks down if there are substantive changes in what's required to build them. Given the nearly 4 years between the early 2010 release of 8.3 and the late 2013 release of 8.22, I'd be surprised if they'll build with the same expression.

I often feel when I’m reading an explanation the author is reluctant to really break something down to the essentials and ELI5 ... which ironically can be conceptualized as the most intellectual way to understand something. Please explain it from as close to first principles as possible. Assume as little as possible.

Maybe it is because the author thinks it is an intrinsically hard concept (as per the intro).

Or maybe this person is just kinda awesome at explaining stuff.

Is Nix really that slow?

I think that's the root of the issue.

You seem to be somewhat confused about how Linux executes programs. An executable is just a file on the disk. It might be entirely self-sufficient or it might use libraries. Libraries are just files on the disk too (they end in .so.number where number is a version number). To load the libraries, when it starts a program, Linux first runs another executable called a dynamic linker (usually ld). Where the dynamic linker looks for the libraries files depends of its configuration. Ld will notably look into the paths listed in the LD_LIBRARY_PATH variable.

All of this has nothing to do with distributions. It's just how Linux works and will always be true. As long as you have all the so your executable need in a place where ld can find them, it will run.

Now, what a package manager does when you install a package is just putting all the files contained within it in the correct location, ensuring you will have installed the necessary shared libraries for the executable you want to run and setting up everything that might need to be setup.

You can manually add executables and so files as much as you like. The only issue you might encounter is using a location which is also used by a package. Depending of the package manager you use, that might either overwrite your modification or make the package installation fail.

While I have never used it, apparently Nix is well behaved and only install things into /nix so that shouldn't be an issue.