The differences between distros are the things you mentioned. They all use the Linux kernel, so the differences are in the DE, installer, theme, default packages, and package manager. These changes come about from design choices: rolling vs versioned releases, stability goals, FOSS vs proprietary packages/repositories, things like systemd vs alternatives, and overall goals/use cases (lightweight, server, etc).
A distro can be as little as a theme change. The famous Hannah Montana Linux is KUbuntu with a custom theme, icon pack, and Hannah Montana as the background.