Nix with nix-output-monitor (nom). https://github.com/maralorn/nix-output-monitor
It shows the tree of packages to download and to build. It shortens the tree in realtime when packages have finished downloading/building and lengthens the tree when it finds more packages it needs to handle. Very fun and satisfying.
I haven't seen this in other package managers.
I don't know exactly how Repology works, but I was interested as well.
This holds what sources are being used for repology in Debian: https://github.com/repology/repology-updater/blob/master/repos.d/deb/debian.yaml This repository seems to be used to merge/split package names: https://github.com/repology/repology-rules
The packages of Debian are split into different subpackages (dev, lib, doc and its base). This happens for Nix as well, but packages there just have different outputs. For instance, openssl has as outputs: bin debug dev doc man out. I don't think repology counts those outputs, so it shouldn't count subpackages as well. I guess these rules are merging these together: https://github.com/repology/repology-rules/blob/master/800.renames-and-merges/openssl.yaml.