NixOS is described as 'GNU/Linux distribution that aims to improve the state of the art in system configuration management. In existing distributions, actions such as upgrades are dangerous: upgrading a package can cause other packages to break, upgrading an entire system is much less' and is a popular Linux Distro in the os & utilities category. There are more than 100 alternatives to NixOS for a variety of platforms, including Linux, Self-Hosted, Mac, BSD and Windows apps. The best NixOS alternative is Linux Mint, which is both free and Open Source. Other great apps like NixOS are Ubuntu, Debian, Fedora and Arch Linux.
they both are the most prominent purely functional package managers.