What the difference between a Linux distro and a unix-based OS?

I wonder why systems like Void, NixOS and GentOO are classified as Linux distros while systems like FreeBSD and are considered as a different operation systems despite allowing similar applications to be installed and run through their own package managers. I wonder what is so different about Solaris and FreeBSD that we don’t call them Linux distros while calling Gentoo and NixOS as a Linux distro despite having very different infrastructure compared to mainstream ones like Ubuntu or Fedora?