Debian and Ubuntu use .deb packages and use apt to install packages. Fedora uses .rpm packages and dnf to install packages. Not sure what Arch uses. Fedora is

Debian and Ubuntu use .deb packages and use apt to install packages. Fedora uses .rpm packages and dnf to install packages. Not sure what Arch uses. Fedora is backed by Red Hat (an American company) while Ubuntu is backed by Canonical (not American). Fedora is more bleeding edge than Ubuntu. Fedora has SELinux with good policies by default and Ubuntu has App-Armor, but I think disabled by default. One would use Fedora when developing software for servers (CentOS / RHEL). Ubuntu, however, has more packages in the repositories, so it is more end-user friendly.