Sailfish OS is a mobile phone operating system based on Linux, and not just the kernel - it’s a full Linux system with a packet manager, systemd services etc. running a wayland compositor.
A few parts - some UI stuff and parts of the Android App Support - are closed-source. And it’s technical Freemium, but a full lifetime license currently costs only €24.90/device and you still get a fully functioning mobile OS for free.
But whoever decides to use it will realize that the company behind it is honest, friendly & dedicated. Yeah I know, it almost sounds corny. But having used it & its forums across 3 devices, since 2020, that’s how I feel. Feelings aside though, they’re also subject to EU law. You can read the legal stuff unde one of the links below.
It’s a successor of the also Linux based Maemo and Meego operating systems developed by Nokia since 2005. After its cellphone branch demise (thanks to Microsoft’s Embrace, Extend, Extinguish tactics) some developers branched off to continue this work => Sailfish OS.
https://forum.sailfishos.org/
https://jolla.com/
https://sailfishos.org/
https://docs.sailfishos.org/
Want to try it on one of the supported devices?
https://docs.sailfishos.org/Support/Supported_Devices/
https://jolla.com/sailfishxinstall/
https://shop.jolla.com/downloads/ (yes, you’ll need to register but it’s free until you decide to pay for it)
It seems that Sony’s Xperia range is fairly well supported by the community; have a look for yourself: https://forum.sailfishos.org/c/hw-adaptation/10