The site also had this very nice description of the update process by @domenkozar, which is why I continued linking it even after it stopped updating. But I guess I can just quote it here:
How does channel update?
1. Git commit
Anyone with commit access can push changes to either master or one of the release-XX.XX branches.
2. Hydra build
Each channel has an associated jobset on Hydra (Nix CI) that is a collection of individual jobs . A job roughly corresponds to building a particular package, which itself may depend on other packages built by other jobs .
Before a channel can update, its associated jobset must be finished building, though the building of some jobs may fail.
As long as certain special jobs – unstable for nixpkgs, and tested for nixos – build successfully, the channel can update. The role of these special jobs is to ensure that essential functionality is present on each channel update.
There are two major differences between those jobsets:
- nixpkgs-unstable
- builds all packages for supported platforms (Linux, Darwin)
- unstable job that consists of most commonly used packages
- nixos-*
- builds all packages and NixOS machinery only for Linux
- tested job that consists of mostly NixOS tests firing up qemu instances with different kinds of configurations
3. Channel update
For a channel to be updated two conditions need to be satisfied:
- Particular jobset evaluation needs to be completely built ie. no more queued jobs, even if some jobs may fail
- Particular jobset evaluation’s tested/unstable job needs to be built succesfully
The nixos.org server has a cronjob for which nixos-channel-scripts are executed and poll for the newest jobset that satisfies the above two conditions and trigger a channel update.
Once triggered, release files such as ISOs are copied. For the NixOS channel command-not-found index is generated, which can take some time since it has to fetch all packages.
nixpkgs is quickly updated since none of the above needs to happen once a channel update is triggered.
Some details are no longer true (e.g. the command-not-found tooling has been replaced by nix-index) but I believe it is still mostly correct.