@civodul As for nixpkgs-review, the heavy lifting is currently done by ofborg, the soon to be discontinued PR CI. (ie. the component evaluating and building the PRs, but not populating the cache/building the channels).

For each PR, ofborg evaluates Nixpkgs and checks the changed paths. It then creates a gist listing the updated packages (for instance https://gist.github.com/GrahamcOfBorg/b65fc0e8791f16ae1a0be898a9b2779d). Nixpkgs-review retrieves this file for the PR we want to review (https://github.com/Mic92/nixpkgs-review/blob/907925df227584ae4c0eb38db51fd23fe495d276/nixpkgs_review/github.py#L89) and locally evaluates/builds the changed packages.

There's also an option to inhibit this behaviour and evaluate everything locally, without using ofborg https://github.com/Mic92/nixpkgs-review/blob/907925df227584ae4c0eb38db51fd23fe495d276/nixpkgs_review/cli/__init__.py#L41.

But in practice, it requires a quite beefy machine to be usable. I personally don't do that on my computers.