Overview of pinning techniques

Observations
There are > 5 ways to handle dependencies in Nix expressions:

• Lookup paths and their various syntaxes (<nixpkgs>, -I, NIX_PATH)
• Channels
• Paths to Git submodules
• Built-in fetchers (fetchurl, fetchTarball, fetchGit, ...)
• Nixpkgs fetchers (pkgs.fetchFromGitHub, ...)
• Flakes
• npins
• niv
• gridlock
• Don't even get me started on modules

I likely forgot some...

Problem

• These are too many
• They have different trade-offs
• None of them is clearly superior, but some are clearly inferior

Approaches
We can't fix it before we understand what's wrong.

Make an overview along the lines of Why are there five ways of getting software?. List features and trade-offs.

Willing to help?
Very likely there is no way around it.

Priorities

Add 👍 to issues you find important.

Start by reviewing the listed pinning techniques and the linked article, then gather the relevant Nix documentation for each approach. Compare their features and trade-offs in an overview, making the scope and conclusions clear; the issue does not name a target file or test.