It adds a launchd service that makes sure stuff sourcing your Nix environment setup script is present in all your shell units on boot, so they get reestablished

It adds a launchd service that makes sure stuff sourcing your Nix environment setup script is present in all your shell units on boot, so they get reestablished when macOS updates nuke them. macOS apps do weird shit to work around macOS quirks. But it's a really good installer. Can't recommend it enough.