An overview of single-purpose Linux distributions

To add, there is also Flatcar Container Linux, the continuation of CoreOS Container Linux (which's parts were reused/further delevoped for Fedora CoreOS). It uses an A/B partition scheme for updating the /usr partition. Like Fedora CoreOS it provides Ignition and Afterburn for configuration.
See https://www.flatcar.org/ or, since I'm one of the maintainers, you can also ask here if you have questions.